xscreensaver: fix build failures

If xdg-open isn't installed (as it wouldn't be in a sandboxed build,
since it wasn't specified as a dependency), this will fail package
checks at the end. In fact, these lines are useless anyway, as .desktop
files (per spec) treat them as comments, not as paths to an interpeter.
This seems to be a misunderstanding upstream.

(gutteridge)