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

# Remove the submodule entry from .git/config
git submodule deinit -f path/to/submodule

# Remove the entry in .gitmodules and remove the submodule directory located at path/to/submodule
git rm -f path/to/submodule

# Remove the submodule directory from the superproject's .git/modules directory
rm -rf .git/modules/path/to/submodule

#Remember to commit the remove change!
git commit -m "removed submodule"

mv subfolder subfolder_tmp
git submodule deinit subfolder
git rm --cached subfolder
mv subfolder_tmp subfolder
git add subfolder

git submodule foreach -q git config -l

Option 1.
Remove .gitmodules file from project

Option 2.
rm -rf .git/modules/path_to_submodule

Remember to commit the remove change!