One of the major changes here is `pos_settle_due` in which is now only
possible to settle invoices and not pos order. That's because when
paying by customer account it will create an invoice each time.

### Removed fields
- `invoice_journal_id` and `pos_invoice_journal_id` because now all
invoice are created with the same journal, the one defined on the pos
config.

- `account_default_pos_receivable_account_id` because now the receivable
account is defined though a default partner on the pos.config

- `cash_real_transaction`, `cash_register_balance_start`,
`cash_register_balance_end_real and `cash_journal_id` all these infos
are now handled though the cash statement and its lines, so no need to
have them on the session

- `is_online_payment` because online payment method is now identified by
type == 'online' and not by a boolean field

- `settle_invoice_product_id`, `deposit_product_id` and
`settle_due_product_id` because we don't create a pos.order when
settling an invoice, so no need to have these products.

- `settled_invoice_id` and `settled_order_id` because we don't create a
pos.order when settling an invoice, so no need to link them anymore.

- `init_customer_due_total`, `customer_due_total`,
`settled_order_line_ids`, `settled_orders_count`, `commercial_partner_id`
because we don't settle pos.order anymore

### Test tracking
A lot of tests were replaced because all accounting entries for order
and session closing are now refactored.

Some tests were also removed because they were not relevant anymore with
the new way of handling payment and invoicing on pos.order.

#### Replaced tests
`test_classic_sale_order` replace:
- `test_state_when_closing_register`

`test_fifo_valuation_with_invoice` replace:
- `test_fifo_valuation_with_invoice_when_pos_customer_is_delivery_type`

`test_fixed_tax_negative_qty_should_be_negative` replace:
- `test_fixed_tax_negative_qty`
- `test_fixed_tax_positive_qty`

`test_user_right_on_statement_line_for_pos_user` replace:
- `test_pos_closing_cash_details`

`test_rounding_when_closing_session` replace:
- `test_invoice_rounding_overpaid_backend`
- `test_pos_create_account_move_round_globally`
- `test_rounding_method`

Moved in `test_pos_accounting.py`:
`test_journal_entries_category_without_account`
`test_tax_change_blocked_when_open_pos_session`
`test_order_partial_refund_rounding`

Test suite in `test_pos_accounting.py` replace:
- `test_pos_simple_invoiced_orders.py`
- `test_orders_no_invoiced`
- `test_orders_with_invoiced`
- `test_01_invoiced_order_from_other_customer`
- `test_02_all_orders_invoiced_mixed_customers`
- `test_total_due.py`
- `test_02_orders_without_invoice`
- `test_03_orders_with_invoice`

`test_invoice_a_negative_order_should_create_credit_note` replace:
- `test_manual_refund_negative_qty_invoice_creates_credit_note`

`test_invoice_an_order_from_closed_session` replace:
- `test_invoice_past_order_affecting_taxes`
- `test_invoice_past_refund`
- `test_invoice_past_order`
- `test_sale_order_postponed_invoicing`
- `test_order_invoiced_after_session_closed`
- `test_order_invoiced_customer_account_after_session_closed`
- `test_sale_order_postponed_invoicing`


`test_pos_order_sale_and_refund_with_taxes` replace:
- `test_orders_with_invoiced`
- `test_return_order`

`test_pos_order_sale_and_refund_with_taxes_invoiced` replace:
- `test_pos_create_correct_account_move`
- `test_return_order_invoiced`

`test_pos_order_sale_and_refund_with_taxes_not_invoiced` replace:
- `test_pos_create_correct_account_move`
- `test_return_order`

`test_invoiced_order_with_discount_sale_and_refund_with_tax` replace:
- `test_pos_create_correct_account_move_round_globally_discount`
- `test_pos_create_correct_account_move_round_globally_discount_real_use_case`


`test_tax_change_blocked_when_open_pos_session` replace:
- `test_tax_is_used_when_in_transactions`

`test_pos_payment_direction_and_account` replace:
- `test_pos_payment_direction_and_accounts`

`test_cash_statement_opening_and_closing_consistency` replace:
- `test_cash_register_if_no_order`
- `test_cash_payments_should_reflect_on_next_opening`

`test_invoicing_zero_amount_pos_order` replace:
- `test_orders_with_zero_valued_invoiced`

`test_order_partial_refund_rounding` replace:
- `test_refund_rounding_backend`
- `test_order_partial_refund`

`test_pos_config_with_other_currency_than_company` replace:
- `test_order_to_payment_currency`

`test_accounting_items_when_closing_with_bank_difference` replace:
- `test_payment_difference_accounting_items`
- `test_state_when_closing_register`

`test_pos_order_with_closing_storno` replace:
- `test_sale_order_postponed_invoicing_storno`

`test_various_orders` replace:
- `test_correct_partner_on_invoice_receivables`

`test_pos_order_session_closing_with_fp` replace:
- `test_01_no_invoice_fpos`
- `test_02_no_invoice_fpos_no_tax_dest`

`test_available_stock_order_no_invoice` replace:
- `test_orders_no_invoiced`
- `test_return_order`

`test_available_stock_order_with_invoice` replace:
- `test_orders_with_invoiced`
- `test_return_order`

`test_pos_order_with_company_branch` replace:
- `test_pos_branch_account`

#### Removed tests

`test_split_payment_linked_to_accounting_partner` split transaction is
now only available when using customer account (type == pay_layer)

`test_pos_hr_session_name_gap` removed not useful

`test_order_to_invoice_no_tax` not useful

`test_order_invoiced_customer_account_after_session_closed` not useful,
customer account payment are always invoiced but this test is almost
the same `test_invoice_an_order_from_closed_session`

`test_start_balance_with_two_pos` useless test, cash payment cannot be
shared.

`test_different_customer_invoices_settlement_same_session_reconciled_separately`
settling an invoice doesn't create `pos.order` anymore

`test_split_cash_payments` removed because split on customer account
is now available only when using `pay_later` payment method.

`test_receivable_account_reconciliation.py`, `test_pos_reconcile` no
more necessary, payment are now directly added to the account.move

`test_pos_settling_account_resets_on_payment_screen_unmount` removed
because now when leaving payment screen it will delete the order if its
a settling one.

`test_pos_settle_due_with_rounding` deleted, wtf we round on a money
deposit ?

`test_split_payment_linked_to_accounting_partner` split_transactions is
no more available with classic pm, only with pay_later.

`test_05_tax_base_amount` removed tax base amount are handled and tested
by accounting helpers

`test_no_kitchen_confirmation_for_deposit_money` removed because deposit
money doesn't create a pos.order anymore.

`test_order_with_deleted_tax` is now not possible to archive a tax used
by an open order.

`fastValidate` hoot deleted in pos_settle_due because isn't possible to
use fast validation with settle customer invoice anymore.

`test_double_syncing_same_order` removed, sync data with the same UUID
will update the same order, but since a payment_ids create command is in
the data, its normal that this payment is created two times.