[IMP] point_of_sale: improve consistencies between models

The deletion of some records can create inconsistencies in the
point_of_sale.

It can create inconsistencies when a session is open, for example if you
delete a partner that can be set on a pos order. You'll also got
problems if you remove an employee and this employee can be used in an
open session.

We also avoid to get point_of_sale config inconsistencies, by deleting
models required by it. If you delete a picking type used in a pos
config or a sequence.

To avoid all this consistencies, we have restricted the deletion of some
records in specific cases.

TASK-ID: 1879971

closes odoo/odoo#35793

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