media: Revert "media: staging/imx: add media device to capture register"

The imx6-specific subdevs that register a capture device will no
longer hold a reference to the media device, so this commit must be
reverted.

This reverts commit 16204b8a.

Signed-off-by: Steve Longerbeam <slongerbeam@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>