media: cx18: remove unused cx18-alsa-mixer

cx18-alsa-mixer functions are not used since commit 4cb565cc
("V4L/DVB: cx18: make it so cx18-alsa-main.c compiles") 9 year later,
let's just remove them.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>