8svx: move pcm_s8_planar decoder to pcm.c

Removes limitation of max 2 channels for pcm_s8_planar decoder by moving
it to more natural place.

AV_CODEC_ID_8SVX_RAW is not used by anything anymore and is going to be
removed.

Signed-off-by: Paul B Mahol <onemda@gmail.com>