media: cx231xx: Remove vidioc_s_ctrl callback

The driver has been converted to the control framework in the past:
commit 88b6ffed ("[media] cx231xx-417: convert to the control
framework").

This function is never called, the core will only use the control
framework instead.

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Tested-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>