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`