media: v4l2-subdev: Drop .set_mbus_config() operation

The .set_mbus_config() operation is deprecated, and nothing in the
kernel uses it. Drop it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>