[REM] mail: remove duplicated method

This commit remove duplicated code probably due to a forward-port of a backport
or something like that. Or maybe due to a cosmic beam.

See commits https://github.com/odoo/odoo/commit/14f80bfae66f2c81abb0b8acc0ac558c60ef1d44 and https://github.com/odoo/odoo/commit/36141931bf9a57d01747cb219fdcfdfd7978ab42.

Cheers.

addons/mail/models/ir_model.py

@@ -52,12 +52,6 @@ class IrModel(models.Model):
             model_class._inherit = parents + ['mail.thread']
         return model_class
 
-    def unlink(self):
-        # Delete followers for models that will be unlinked.
-        query = "DELETE FROM mail_followers WHERE res_model IN %s"
-        self.env.cr.execute(query, [tuple(self.mapped('model'))])
-        return super(IrModel, self).unlink()
-
 
 class IrModelField(models.Model):
     _inherit = 'ir.model.fields'