[IMP] mail: avoid getting field description

When calling track_prepare, an important part of the logic is getting
description in fields_get. We actually don't need the description,
fields_get is mainly use here to check for groups, but the dictionnary
is immediately transformed to a set making values irrelevant.

The same optimisation is done in _message_track, usefull to avoid an
additionnal query in test_recurring_order_creation_perf, because of a
value not in cache.

closes odoo/odoo#132950

Related: odoo/enterprise#46997
Signed-off-by: Xavier Dollé (xdo) <xdo@odoo.com>