submodule="src/fields/builder"
git rm "$submodule" && rm -rf ".git/modules/$submodule" && git config -f ".git/config" --remove-section "submodule.$submodule" 2> /dev/null && git commit -m "Remove submodule $submodule"
Credit: https://gist.github.com/myusuf3/7f645819ded92bda6677#gistcomment-2797195