V4L/DVB (5998): ivtv: no need to mute the audio input

When changing channels the audio has to be muted. This is done
by calling CX2341X_ENC_MUTE_AUDIO and by muted the audio input.
The latter is not necessary and is now removed.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>