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:
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.