Remove redundant declaration for XidInMVCCSnapshot

This was added for no good reason by c91560defc, after b7eda3e0e3
had just moved the prototype from utils/tqual.h to utils/snapmgr.h.

Author: Japin Li <japinli@hotmail.com>
Discussion: https://postgr.es/m/MEYP282MB16693A409F3282A9DB287BADB63E9@MEYP282MB1669.AUSP282.PROD.OUTLOOK.COM

src/include/access/heapam.h

@@ -213,7 +213,6 @@ extern HTSV_Result HeapTupleSatisfiesVacuumHorizon(HeapTuple htup, Buffer buffer
 extern void HeapTupleSetHintBits(HeapTupleHeader tuple, Buffer buffer,
 								 uint16 infomask, TransactionId xid);
 extern bool HeapTupleHeaderIsOnlyLocked(HeapTupleHeader tuple);
-extern bool XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot);
 extern bool HeapTupleIsSurelyDead(HeapTuple htup,
 								  struct GlobalVisState *vistest);