Remove games/include/udmfprops_zdoom.cfg.

ZDoom isn't a game, nothing includes this file, and it's identical to
ports/include/udmf/props_zdoom.cfg.
Remove games/include/udmfprops_boom.cfg.

Same story: not actually a game, unused, and duplicate of another file.
Remove games_old.

These files conflate games with ports, don't seem to be loaded by the
app anywhere, and haven't been touched in a year and a half so are
surely way out of date anyway.
Remove a whole bunch of unused config files.

Turns out all of these were renamed ages ago, but the originals have
lingered.
Merge pull request #123 from eevee/game-config-cleanup

Delete unused game config files.