Uninitialize git repository

Just deleting the .git directory stored in the repository will pretty much do it.

git stores all the repository related data in a folder named ".git" inside your repository folder. So you just want to keep your files but change it into a "not-git-repository". Warning: this is irreversible!

cd path/to/repo
rm -rf .git

The only files that might remain are hidden ".gitignore" files (but only if you added them yourself or using some tool like eclipse). To remove them:

find . -name ".gitignore" | xargs rm