[FIX] base: avoid triggering recomputations based on deleted fields

After deleting records from model 'ir.model.fields', make sure to not
trigger recomputations based on those fields, as they have been removed
from the database schema.

Also shorten the deletion of 'ir.model.fields.selection' records: as
they are deleted after 'ir.model.fields', most of them have already been
deleted on cascade.

closes odoo/odoo#39600

Signed-off-by: Adrian Torres (adt) <adt@odoo.com>