.gitmodules: Remove empty .gitmodules file

This should have happened in:

  commit e607ec335d6b60263014e1e02936dc0a1ae1df7a
  Author: Simon Wex <simon@simonwex.com>
  Date:   Wed Feb 1 16:32:30 2012 -0800

    Removing full bootstrap project
Merge pull request #1 from wking/remove-.gitmodules

.gitmodules: Remove empty .gitmodules file