Remove obsolete nbtree relation extension comment.

Commit 0d1fe9f7 improved the approach that vacuumlazy.c takes when it
encounters an empty heap page.  It no acquires the relation extension
lock.

src/backend/access/nbtree/nbtree.c

@@ -963,11 +963,10 @@ btvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
 	 * recycled.  Taking the lock synchronizes things enough to prevent a
 	 * problem: either num_pages won't include the new page, or _bt_getbuf
 	 * already has write lock on the buffer and it will be fully initialized
-	 * before we can examine it.  (See also vacuumlazy.c, which has the same
-	 * issue.)	Also, we need not worry if a page is added immediately after
-	 * we look; the page splitting code already has write-lock on the left
-	 * page before it adds a right page, so we must already have processed any
-	 * tuples due to be moved into such a page.
+	 * before we can examine it.  Also, we need not worry if a page is added
+	 * immediately after we look; the page splitting code already has
+	 * write-lock on the left page before it adds a right page, so we must
+	 * already have processed any tuples due to be moved into such a page.
 	 *
 	 * We can skip locking for new or temp relations, however, since no one
 	 * else could be accessing them.