Remove mod settings option

Deleting ~/.minetest/worlds/*/world.mt will break a world if it uses a game other then minetest or a back end other then sqlight3 (the defaults). Currently you can somewhat safely (somewhat safely because you're removing a mod, no matter how you remove a mod, there can be leftover 'unknown nodes' in the world from the mod) clear each line except "gameid = <something>"and "backend = <something>"; More important lines like those two are planned in the future. But I don't know how this could be implemented in CleanerML