vss: Remove self-made prefaulting.

vss: Remove self-made prefaulting.

It used to help a bit on a very old laptop with broken IDE hardware,
see commit 7bba6232 (vss: Mmap audio files using MAP_POPULATE) from
six years ago. But even there it did not eliminate buffer underruns
completely. Our prefault code is too ugly to live, and its benefits
are questionable at best, so let's get rid of it.

We still map the audio file with MAP_POPOLATE, which should be enough
get the full audio file cached before chunks are sent.