btrfs: remove unused ulist members

Commit "btrfs: ulist: Add ulist_del() function" (d4b80404)
removed some debugging code but left the structure defintions.

Reviewed-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>