Fix the semantic of the new List.remove_assoc_f

 Unlike Ocaml's stdlib List.remove_assoc, I was raising Not_found
 when the key to remove wasn't there...

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16931 85f007b7-540e-0410-9357-904b9bb8a0f7
Restore my version of notation_ops.ml now that List.remove_assoc_f is fixed

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16932 85f007b7-540e-0410-9357-904b9bb8a0f7