Removing some lone List.assoc & List.mem in lib.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16738 85f007b7-540e-0410-9357-904b9bb8a0f7
Fixing bug #3083.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16739 85f007b7-540e-0410-9357-904b9bb8a0f7