vd_lavc: remove explicit crystalhd support

This removes "--hwdec=crystalhd".

I doubt anyone even tried to use this. But even if someone wants to
use it, the decoders can still be explicitly invoked with e.g.:

    --vd=lavc:h264_crystalhd

The only advantage our special code provided was fallback to
software decoding. (But I'm not sure how the ffmpeg crystalhd
pseudo-decoder actually behaves.)

Removing this will allow some simplifications as soon as we don't need
vdpau_old.c anymore.