Remove SYNC_REP_DISABLED state

This state was meant to prevent auxiliary processes from waiting from syncrep.
As far as we can tell, auxiliary process should not be calling
SyncRepWaitForLSN() unless it is committing a transaction.  And if it is
committing a transaction, it better wait for mirror to flush XLOG upto the
commit LSN.  One example of an auxiliary process committing a transaction is
FTS, when it updates cluster configuration on master.
Co-authored-by: NTaylor Vesely <tvesely@pivotal.io>

src/backend/replication/syncrep.c

@@ -116,21 +116,6 @@ SyncRepWaitForLSN(XLogRecPtr XactCommitLSN)
 		return;
 	}
 
-	/*
-	 * XXX: Some processes (e.g. auxiliary processes) don't need even
-	 * check the walsenders.  We just stay away from any of the shared
-	 * memory values.  When we move to fully-synchronous replication,
-	 * any xlog change needs to be considered to be synchronous,
-	 * but for now this is acceptable.
-	 */
-	if (MyProc->syncRepState == SYNC_REP_DISABLED)
-	{
-		elogif(debug_walrepl_syncrep, LOG,
-				"syncrep wait -- Not waiting for syncrep as synrep state for this process is disabled.");
-
-		return;
-	}
-
 	if (GpIdentity.segindex != MASTER_CONTENT_ID)
 	{
 		/* Fast exit if user has not requested sync replication. */

src/backend/storage/lmgr/proc.c

@@ -597,13 +597,6 @@ InitAuxiliaryProcess(void)
 	for (i = 0; i < NUM_LOCK_PARTITIONS; i++)
 		SHMQueueInit(&(MyProc->myProcLocks[i]));
 
-	/*
-	 * Auxiliary process doesn't bother with sync rep.  Though it was
-	 * originally supposed to not do transaction work, but it does in GPDB,
-	 * we mark it and avoid sync rep work.
-	 */
-	MyProc->syncRepState = SYNC_REP_DISABLED;
-
 	/*
 	 * Acquire ownership of the PGPROC's latch, so that we can use WaitLatch.
 	 * Note that there's no particular need to do ResetLatch here.

src/include/replication/syncrep.h

@@ -27,7 +27,6 @@
 #define NUM_SYNC_REP_WAIT_MODE	2
 
 /* syncRepState */
-#define SYNC_REP_DISABLED			-1
 #define SYNC_REP_NOT_WAITING		0
 #define SYNC_REP_WAITING			1
 #define SYNC_REP_WAIT_COMPLETE		2