media: ivtv: remove ivtv-alsa-mixer

ivtv-alsa-mixer functions was introduced in commit 269c11fb
("[media] ivtv, ivtv-alsa: Add initial ivtv-alsa interface driver for ivtv")
But according to commit message, ivtv-alsa-mixer.c was already dead code.
5 years after, we should remove it.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
[hans.verkuil@cisco.com: removed forgotten ivtv-alsa-mixer.h include]
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>