staging: media: imx: Drop imx_media_subdev_bound()

The imx_media_subdev_bound() function is a no-op, and the async notifier
.bound() operation is optional. Drop it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>