There isn't any real use of following members of struct btrfs_root
so delete them.

struct kobject root_kobj;
struct completion kobj_unregister;
Signed-off-by: NAnand Jain <anand.jain@oracle.com>
Reviewed-by: NDavid Sterba <dsterba@suse.cz>
Signed-off-by: NChris Mason <clm@fb.com>