media: imx7-mipi-csis: remove subdev_notifier

It was defined a notifier in the csi_state structure that is never
allocated. And besides that it's unregister in the remove, even though
it is a no-op, just remove both.

Fixes: 7807063b ("media: staging/imx7: add MIPI CSI-2 receiver subdev for i.MX7")
Reported-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Suggested-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Reviewed-by: Steve Longerbeam <slongerbeam@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>