[media] bttv: remove g/s_audio since there is only one audio input

Note that the current driver does not implement enumaudio (so apps cannot
tell that audio inputs are present), it does not set V4L2_CAP_AUDIO, nor
does it set audioset when calling ENUM_INPUT. And G_AUDIO doesn't set the
stereo flag either. So these g/s_audio ioctls are quite pointless and
misleading. Especially since some surveillance boards do not have audio
at all.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>