Delete unused game config files.

I'm a little unsure about this, but as far as I can tell, none of these files are used any more.

- `games_old` is a backup copy of all the configuration from before the `games`/`ports` split.
- Files like `things/hexen_things.cfg` have long since been moved (er, copied?) to `things_hexen.cfg`.
- `udmfprops_*.cfg` have their own dedicated `udmf/` directory now.

I grepped for every single one of these filenames, and none of them are actually `#include`d anywhere.  Poked around the map editor a bit and it also seems unaffected.