Remove WARNING message from brin_desummarize_range

This message was being emitted on the grounds that only crashed
summarization could cause it, but in reality even an aborted vacuum
could do it ... which makes it way too noisy, particularly since it
shows up in regression tests and makes them die.

Reported by Tom Lane.
Discussion: https://postgr.es/m/489091.1593534251@sss.pgh.pa.us

src/backend/access/brin/brin_revmap.c

@@ -333,7 +333,6 @@ brinRevmapDesummarizeRange(Relation idxrel, BlockNumber heapBlk)
 	OffsetNumber revmapOffset;
 	OffsetNumber regOffset;
 	ItemId		lp;
-	BrinTuple  *tup;
 
 	revmap = brinRevmapInitialize(idxrel, &pagesPerRange, NULL);
 
@@ -365,6 +364,10 @@ brinRevmapDesummarizeRange(Relation idxrel, BlockNumber heapBlk)
 	regBuf = ReadBuffer(idxrel, ItemPointerGetBlockNumber(iptr));
 	LockBuffer(regBuf, BUFFER_LOCK_EXCLUSIVE);
 	regPg = BufferGetPage(regBuf);
+	/*
+	 * We're only removing data, not reading it, so there's no need to
+	 * TestForOldSnapshot here.
+	 */
 
 	/* if this is no longer a regular page, tell caller to start over */
 	if (!BRIN_IS_REGULAR_PAGE(regPg))
@@ -386,24 +389,13 @@ brinRevmapDesummarizeRange(Relation idxrel, BlockNumber heapBlk)
 		ereport(ERROR,
 				(errcode(ERRCODE_INDEX_CORRUPTED),
 				 errmsg("corrupted BRIN index: inconsistent range map")));
-	tup = (BrinTuple *) PageGetItem(regPg, lp);
-	/* XXX apply sanity checks?  Might as well delete a bogus tuple ... */
-
-	/*
-	 * We're only removing data, not reading it, so there's no need to
-	 * TestForOldSnapshot here.
-	 */
 
 	/*
-	 * Because of ShareUpdateExclusive lock, this function shouldn't run
-	 * concurrently with summarization.  Placeholder tuples can only exist as
-	 * leftovers from crashed summarization, so if we detect any, we complain
-	 * but proceed.
+	 * Placeholder tuples only appear during unfinished summarization, and we
+	 * hold ShareUpdateExclusiveLock, so this function cannot run concurrently
+	 * with that.  So any placeholder tuples that exist are leftovers from a
+	 * crashed or aborted summarization; remove them silently.
 	 */
-	if (BrinTupleIsPlaceholder(tup))
-		ereport(WARNING,
-				(errmsg("leftover placeholder tuple detected in BRIN index \"%s\", deleting",
-						RelationGetRelationName(idxrel))));
 
 	START_CRIT_SECTION();