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 "
 - Delete the now untracked submodule files rm -rf path_to_submodule
 
Excellent!