Removing the use of Fset/Fmap from Trie. There was actually only
one place depending on fold order, which was easy to bypass.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16514 85f007b7-540e-0410-9357-904b9bb8a0f7
Removing Fset, since it is not used anymore.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16515 85f007b7-540e-0410-9357-904b9bb8a0f7