Fix a broken gitlink left behind by deletion of .gitmodules

When attempting to install this vim plugin using Vundle, I encountered an the following error with the `git clone`:

    No submodule mapping found in .gitmodules for path 'plugin/editorconfig-core-py/tests'

This was relatively easy to fix after I read up on the error at [StackOverflow]
(http://stackoverflow.com/questions/4161022/git-how-to-track-untracked-content/4162672#4162672).

All I had to do was `git rm --cached` the offending directory.