Remove the gitblit-groovy submodule

Submodules do not work with r10k
Remove empty .gitmodules file after removing last submodule
Squashed 'files/gitblit-groovy/' content from commit 5aca39c

git-subtree-dir: files/gitblit-groovy
git-subtree-split: 5aca39c703fd40ce83d9cb2fb9f3c567ac0c55bc
Merge commit 'ea441f97d194a01c34545de29f0d1cc65eca2c9c' as 'files/gitblit-groovy'