-Rsn removes the package, the dependencies not needed by other packages and it’s configuration files in /etc (supposedly).
If there are still remnants of the package around, the best way to remove those is to re-install the package and remove it, like:
sudo pacman -S minetest --overwrite "*"
sudo pacman -Rsn minetest
This will re-install the package, making pacman aware of the files that where left behind, and then remove it with those files.