[media] cx231xx: remove broken audio input support from the driver

The audio selection code is broken. Audio and video indices were
mixed up and s_audio would reject changing the audio input to
something else anyway, so what's the point?
All the audio input code has been removed.

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