[FIX] mail: fix mismatch between recordset and id

Recently a ref() has been converted into a direct xml id match. As this
does not return a recordset but an ID, the .id that follows is not
necessary anymore.

addons/mail/models/mail_thread.py

@@ -474,11 +474,11 @@ class MailThread(models.AbstractModel):
         msg_comment = MailMessage.search([
             ('model', '=', self._name),
             ('res_id', '=', self.id),
-            ('subtype_id', '=', subtype_comment.id)])
+            ('subtype_id', '=', subtype_comment)])
         msg_not_comment = MailMessage.search([
             ('model', '=', self._name),
             ('res_id', '=', self.id),
-            ('subtype_id', '!=', subtype_comment.id)])
+            ('subtype_id', '!=', subtype_comment)])
 
         # update the messages
         msg_comment.write({"res_id": new_thread.id, "model": new_thread._name})