Have you read the instructions on the upstream GitHub site used?
It says:
When OpenVPN is installed, you can run the script again, and you will get the choice to:
Add a client Remove a client Uninstall OpenVPN
I don’t know if that works, but it seems the most viable way.
A suggestion for the future: when you don’t want or can’t use official packages in the repos or AUR, always make sure first that there is an uninstall script or procedure.