media: v4l2-subdev: Drop outdated comment for v4l2_subdev_pad_config

The v4l2_subdev_pad_config structure is not passed to subdev operations
anymore. Drop an outdated comment to refers to that old mechanism.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>