staging: media: imx: Drop unused helper functions

The imx_media_find_subdev_by_fwnode(),
imx_media_find_subdev_by_devname(), imx_media_pipeline_csi2_channel()
and imx_media_pipeline_video_device() helper functions are not used.
Drop them.

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>