[FIX] payment: properly uninstall if transaction exists

Before this commit:
    - Install the `payment` module
    - Generate a `payment.transaction`
    - Try to uninstall the `payment` module
    -> Uninstall process crashes and the DB is rolled back

This happens because the field `acquirer_id` of the model
`payment.transaction` is a required many2one with ondelete='set null'
(the default), this of course makes no sense, and when the ORM tries to
set the field of these records to NULL, it crashes.

This has been fixed globally in saas-12.2 at ee6dc40a but for stable
versions it is best to simply simulate the behavior of an
ondelete='cascade' which is what this commit does via an uninstall_hook

Fixes #37379

closes odoo/odoo#37422

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>