[media] v4l2-ctrls: remove unclaimed v4l2_ctrl_add_ctrl() interface

v4l2_ctrl_add_ctrl() interface has no users since its introduction in
commit 0996517c

 ("V4L/DVB: v4l2: Add new control handling framework")
and its functionality is covered by v4l2_ctrl_new() and derivative
interfaces, so it is safe to remove the interface from the kernel.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>