[REM] maintenance: remove dead code about responsible auto subscription

An override related to mail.thread auto subscription has been added in
maintenance application at 23a2bac3fa in 9.0. Since that version
however the field has been renamed to avoid confusion with user_id that is
generally a responsible with access rights to handle the document.

This code is therefore not necessary anymore and can be safely removed
without impacting any functional flow.

addons/maintenance/models/maintenance.py

@@ -217,15 +217,6 @@ class MaintenanceEquipment(models.Model):
             self.message_subscribe_users(user_ids=[vals['owner_user_id']])
         return super(MaintenanceEquipment, self).write(vals)
 
-    @api.model
-    def _message_get_auto_subscribe_fields(self, updated_fields, auto_follow_fields=None):
-        """ mail.thread override so user_id which has no special access allowance is not
-            automatically subscribed.
-        """
-        if auto_follow_fields is None:
-            auto_follow_fields = []
-        return super(MaintenanceEquipment, self)._message_get_auto_subscribe_fields(updated_fields, auto_follow_fields)
-
     @api.model
     def _read_group_category_ids(self, categories, domain, order):
         """ Read group customization in order to display all the categories in