[18.0][MIG] pos_transfer_account: Migration to 18.0

[P-H-Phuc]

This module was initially part of the Foodcoop set of addons. We're now migrating it to 18.0 and taking this occasion to contribute it to OCA.

• This module was written to extend the functionality of odoo Point Of Sale about transfer account.
• This module is usefull only in a multi Point of Sale context of multi company context.
• With this module, it is now allowed to define for each PoS config a transfer account.
• If this account is set, it will be used when user realizes "Cash In/Out" operation.

• Remove pos_transfer_account/models/cash_box_in.py and pos_transfer_account/models/cash_box_out.py: its model was removed from 14.0.