Steps to reproduce:
1) Via Studio, modify the 'Journal Items' tab to add 'Start date' and 'End date' to the list view
2) Create a group tax (purchase type - see joined screenshots for extra info)
3) Create a bill with the group tax created in step 2)
4) Select deferred dates in the future
5) Check the journal items (two lines with deferred dates)
6) Go back to the 'Invoice lines' tab, delete the deferred dates and check the journal items again => one line still present the deferred dates!!

Reason:
When removing the `deferred_start_date` and the `deferred_end_date` from the invoice line, the newly computed `tax_key` for the tax line(s) will not have the `deferred_start_date` and the `deferred_end_date` fields, by  [_get_deferred_tax_key](https://github.com/odoo-dev/enterprise/blob/c2c3a8591b3a5dfa402acb981564e46e0ff4ced9/account_accountant/models/account_move.py#L582).

However, since when updating the tax line(s), we merge the old values with the new values (obtained in part from the before mentioned updated key), by this code in [_sync_dynamic_line](https://github.com/odoo/odoo/blob/f3d3e0c612392f1a35b9c131d29b4f3bd70a9d36/addons/account/models/account_move.py#L2599-L2602), and since the new key values of the tax line(s) does not have the `deferred_start_date` and the `deferred_end_date` explicitly set to `False`, and since the old key values have those fields set, merging new values with the old values will keep the old `deferred_start_date` and the `deferred_end_date`, resulting in a mismatch between the deferred dates on the invoice line and the tax lines.

Fix:
Now, `_get_deferred_tax_key` explicitly set `deferred_start_date` and the `deferred_end_date` to `False`, to make sure they override any old `deferred_start_date` and the `deferred_end_date` existing in the old keys.

opw-4328465