To remove a submodule you need to:

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