nixos/udev: Always fail if rules contain FHS paths

Partially reverts the following commits:

  9f2a61c5
  9c13fe66



As @edolstra pointed out, it would make more sense to do this by default
instead of having that allowImpurePaths option. This of course might
break systems which add extra packages to udev, but on the upside it's
hard to miss one of these paths now because it won't get buried in the
ocean of build output lines.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>