remove a git submodule

参考: http://stackoverflow.com/questions/1260748/how-do-i-remove-a-submodule

1. Delete the relevant section from the .gitmodules file.
2. Stage the .gitmodules changes git add .gitmodules
3. Delete the relevant section from .git/config.
4. Run git rm --cached path_to_submodule (no trailing slash).
5. Run rm -rf .git/modules/path_to_submodule
6. Commit git commit -m "Removed submodule <name>"
7. Delete the now untracked submodule files
      rm -rf path_to_submodule