Remove AtSubStart_Notify.

Allocate notify-related state lazily instead. This makes trivial
subtransactions noticeably faster.

Patch by me, reviewed and tested by Dilip Kumar, Kyotaro Horiguchi,
and Jeevan Ladhe.

Discussion: https://postgr.es/m/CA+TgmobE1J22S1eC-6N-je9LgrcwZypkwp+zH6JXo9mc=4Nk3A@mail.gmail.com

Author: robertmhaas

src/backend/access/transam/xact.c

@@ -4743,7 +4743,6 @@ StartSubTransaction(void)
 	 */
 	AtSubStart_Memory();
 	AtSubStart_ResourceOwner();
-	AtSubStart_Notify();
 	AfterTriggerBeginSubXact();
 
 	s->state = TRANS_INPROGRESS;