staging: media: imx: Drop IMX_MEDIA_GRP_ID_CSI

The IMX_MEDIA_GRP_ID_CSI group ID isn't used anymore. 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>