[IMP] point_of_sale,pos_mercury: do not save pending payments

It is by default in pos_mercury that pending payments are removed before
syncing the order to the backend. However, this is not the case for the
newly introduced API for payment terminals in pos. In this commit, we
are implementing this pos_mercury feature as default for all terminal
payment methods.

closes odoo/odoo#35579

Task-id: 1984690
Signed-off-by: pimodoo <pimodoo@users.noreply.github.com>