Implemented a safe way to delete constraints, as these constraints didn't exist. It's most likely got to do with the PG upgrade from 7.2 to 7.3 not recording constraints properly. This is a workaround, not a fix. See bug #1294.
the upgrade failed because notification_user_map constraint notif_user_map_notif_id_fk needed the pk on notifications_bak, now drops it and recreates it after the table is reloaded