In case you need to uninstall… be aware that a kitten will die if you do so.
# Remove the submodule
git rm --cached folder-it-was-cloned-into
# We also need to remove the entry from our config file...
git config -f .git/config --remove-section folder-it-was-cloned-into
# ...as well as from the .gitmodules file
git config -f .gitmodules --remove-section folder-it-was-cloned-into
# ...and the modules folder
rm -rf .git/modules/folder-it-was-cloned-into