Fix misleading comment in nodeIndexonlyscan.c.

The stated reason for acquiring predicate locks on heap pages hasn't
existed since commit c01262a8, so fix the comment.  Perhaps in a later
release we'll also be able to change the code to use tuple locks.

Back-patch all the way.

Reviewed-by: Ashwin Agrawal
Discussion: https://postgr.es/m/CAEepm%3D2GK3FVdnt5V3d%2Bh9njWipCv_fNL%3DwjxyUhzsF%3D0PcbNg%40mail.gmail.com

src/backend/executor/nodeIndexonlyscan.c

@@ -179,11 +179,8 @@ IndexOnlyNext(IndexOnlyScanState *node)
 					 errmsg("lossy distance functions are not supported in index-only scans")));
 
 		/*
-		 * Predicate locks for index-only scans must be acquired at the page
-		 * level when the heap is not accessed, since tuple-level predicate
-		 * locks need the tuple's xmin value.  If we had to visit the tuple
-		 * anyway, then we already have the tuple-level lock and can skip the
-		 * page lock.
+		 * If we didn't access the heap, then we'll need to take a predicate
+		 * lock explicitly, as if we had.  For now we do that at page level.
 		 */
 		if (tuple == NULL)
 			PredicateLockPage(scandesc->heapRelation,