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

src/backend/access/transam/xact.c

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

src/include/commands/async.h

@@ -40,7 +40,6 @@ extern void Async_UnlistenAll(void);
 extern void PreCommit_Notify(void);
 extern void AtCommit_Notify(void);
 extern void AtAbort_Notify(void);
-extern void AtSubStart_Notify(void);
 extern void AtSubCommit_Notify(void);
 extern void AtSubAbort_Notify(void);
 extern void AtPrepare_Notify(void);