Remove git-gutter-fringe faces

As of closing the merge request
https://github.com/syohex/emacs-git-gutter-fringe/pull/13 (which is now
available on melpa), the git-gutter-fringe faces now inherit from
git-gutter, so there's no need to have duplicate face declarations
anymore.
Merge pull request #32 from tanzoniteblack/rm-git-gutter-fringe

Remove git-gutter-fringe faces