Unify git.sample.config and git.sample.fhs.config.

There's no point to having two copies of what's essentially the same
file.  In the long term, this means the distributions will need to
patch out cfg.py directories instead of plinth.sample.fhs.config.  So,
if diff plinth.sample.config plinth.sample.fhs.config reveals
anything relevant, put that in your patch.