Remove incorrect assertion in clog.c

We must advance the oldest XID that can be safely looked up in clog
*before* truncating CLOG, and the oldest XID that can't be reused
*after* truncating CLOG.  This assertion, and the accompanying
comment, are confused; remove them.

Reported by Neha Sharma.

Discussion: http://postgr.es/m/CANiYTQumC3T=UMBMd1Hor=5XWZYuCEQBioL3ug0YtNQCMMT5wQ@mail.gmail.com

src/backend/access/transam/clog.c

@@ -678,10 +678,6 @@ TruncateCLOG(TransactionId oldestXact, Oid oldestxid_datoid)
 	 */
 	AdvanceOldestClogXid(oldestXact);
 
-	/* vac_truncate_clog already advanced oldestXid */
-	Assert(TransactionIdPrecedesOrEquals(oldestXact,
-										 ShmemVariableCache->oldestXid));
-
 	/*
 	 * Write XLOG record and flush XLOG to disk. We record the oldest xid
 	 * we're keeping information about here so we can ensure that it's always