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
  ```
- Run 
  ```
  rm -rf .git/modules/path_to_submodule
  ```
- Commit 
  ```
  git commit -m "Removed submodule <name>"
  ```
- Delete the now untracked submodule files 
  ```
  rm -rf path_to_submodule
  ```