media: mc: delete redundant code in __media_device_unregister_entity

media_gobj_destroy has already set graph_obj.mdev to NULL. There is no
need to set it again.

[Sakari Ailus: Remove extra newline, rewrap commit msg]

Signed-off-by: Hangyu Hua <hbh25y@gmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>