Revert "Ordered evars by level of dependency in the merging phase of unification"

Though this commit provides with more "natural" instances of metas
found by unification, it breaks a few contribs. The best approach to
try is maybe now to instead plug apply on evarconv.ml, rather than on
unification.ml!

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16191 85f007b7-540e-0410-9357-904b9bb8a0f7