[REM] stock: remove rounding_method context

Before 9920f20, the
`_compute_product_qty` (`stock.move`) used the context to choose
the rounding method. It was remove (always used 'HALF-UP')
and replace by a comment. The rouding method used by the context was
used only on the `_split` method with always 'HALF-UP'.

Then remove rouding_method of context and remove the obselete comments.

[REM] stock: useless fields

- On `stock.picking`, remove unused `rate_picking_late` and
`rate_picking_backorders` and change the compute accordly
- On `stock.warehouse`, remove unused field warehouse_count and
remove show_resupply which can by replace by
the groups="group_stock_multi_warehouses" since 7bf064f

[REM] mrp: remove unused fields

- On `mrp.production`, remove `workorder_done_count`
unused since mrp v14
- On `mrp.workorder`, remove `use_create_components_lots`,
unused since ed7012e

[REM] stock: remove unused view

Remove `view_picking_type_list`

[REF] purchase: little refactor purchase dashboard

To avoid dynamically building xml_id names,
in the purchase dashboard, remove the name attr used to
construct dynamically the action xml_id.
In any case, it was all the same, then just put it in the js.

odoo/enterprise#12553
odoo/upgrade#1704
task-2041971