Strip out CPU affinity hack.

This was added a long time ago, supposedly as a workaround for
SDL_mixer issues. It's not clear that this ever properly solved the
problem, and it seems unlikely that it's doing any good any more.
Quasar has pointed out that it actually impedes performance, and
some forks may want to use SMP (Strife: Veteran Edition had to
remove it).

This fixes #484 (thanks Quasar).