[media] mx2_camera: Remove 'buf_cleanup' callback

All necessary tasks to end the streaming properly are
already implemented in mx2_stop_streaming() and nothing
remains to be done in this callback.
Furthermore, it only included debug messages so it can
be removed.

Signed-off-by: Javier Martin <javier.martin@vista-silicon.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>