Remove incorrect Assert introduced in c8aeaf3ab.

Already removed incidentally in version 15 (c4649cce3), so this commit
is only applied to versions 13 and 14.

The comment above is misleading in all versions 13 and later, so that
will be fixed in a separate commit.

Discussion: https://postgr.es/m/cfd84cb8-12fe-433a-a4bb-f460a4515f9c.zhaotinghai.zth%40alibaba-inc.com
Reported-by: Tinghai Zhao
Backpatch-through: 13

src/backend/utils/sort/logtape.c

@@ -1271,13 +1271,5 @@ LogicalTapeTell(LogicalTapeSet *lts, int tapenum,
 long
 LogicalTapeSetBlocks(LogicalTapeSet *lts)
 {
-#ifdef USE_ASSERT_CHECKING
-	for (int i = 0; i < lts->nTapes; i++)
-	{
-		LogicalTape *lt = &lts->tapes[i];
-
-		Assert(!lt->writing || lt->buffer == NULL);
-	}
-#endif
 	return lts->nBlocksWritten - lts->nHoleBlocks;
 }