Returns obsolete users IDs to unaward.

def get_obsolete_user_ids(self, db_read=None):
        """
        Returns obsolete users IDs to unaward.
        """
        db_read = db_read or self.db_read

        already_awarded_ids = self.get_already_awarded_user_ids(db_read=db_read, show_log=False)
        current_ids = self.get_current_user_ids(db_read=db_read)
        obsolete_ids = list(set(already_awarded_ids) - set(current_ids))
        obsolete_ids_count = len(obsolete_ids)

        logger.debug(
            '→ Badge %s: %d users need to be unawarded',
            self.slug,
            obsolete_ids_count)

        return (obsolete_ids, obsolete_ids_count)