[FIX] models: do not delete `ir.model.fields` in `_field_create` (#13565)

Following 5c5d0865, apparently unused records
are simply deleted after updating existing ones.  This causes an issue when a
module is installed or upgraded.  The inspection of `ir.model.fields` records
is based on a *partial* registry (not all modules loaded); therefore records
will be deleted and recreated afterwards.  This triggers inconsistencies in
other models that refer to `ir.model.fields`, like server actions, base action
rules, mail templates, etc.: many2one fields will be set to null!