remove length_app_comm from rewrite hint database

Author: NicholasBHubbard

theories/Lists/List.v

@@ -3980,7 +3980,6 @@ Global Hint Rewrite
   length_map (* length (map f l) = length l *)
   length_seq (* length (seq start len) = len *)
   length_app (* length (l ++ l') = length l + length l' *)
-  length_app_comm (* length (l ++ l') = length (l' ++ l) *)
   length_rev (* length (rev l) = length l *)
   app_nil_r (* l ++ nil = l *)
   : list.