[IMP] mail,*: remove useless field `is_master_data`

This field is a non-sense. By using the same logic, it should be added
to any low level model.
Replace the check by a simple verification of presence of an XMLID.

A more generic opt-in solution should be integrated into ORM.

Partially revert commits 0db0e66e and
bbd64c22.

See #29257 odoo/enterprise#3550

closes odoo/odoo#31778

Signed-off-by: Christophe Simonis <chs@odoo.com>