avcodec: remove the VLC DSP mask (except on Android on ARM)

libavutil will always check for all CPU features that it understands,
even those VLC masked. Thus masking features provides no savings.

libavutil runs more tests than VLC. Depending on the platform and
operating system combination, libavutil is either as conservative or
more conservative than VLC. As such masking features provides no extra
safety.

So overall, GetVlcDspMask() is useless, at least nowadays.

Android ARM NEON seems controversial so it is kept for now.