Remove obsolete btrescan() comment.

"Ordering stuff" refered to a _bt_first() call to _bt_orderkeys().
However, the _bt_orderkeys() function was renamed to
_bt_preprocess_keys() by commit fa5c8a05.

_bt_preprocess_keys() is directly referenced just after the removed
comment already, which seems sufficient.

src/backend/access/nbtree/nbtree.c

@@ -440,8 +440,7 @@ btrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
 	}
 
 	/*
-	 * Reset the scan keys. Note that keys ordering stuff moved to _bt_first.
-	 * - vadim 05/05/97
+	 * Reset the scan keys
 	 */
 	if (scankey && scan->numberOfKeys > 0)
 		memmove(scan->keyData,