btrfs: remove unused ulist members

Commit "btrfs: ulist: Add ulist_del() function" (d4b8040459)
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>

fs/btrfs/ulist.h

@@ -19,9 +19,6 @@
  *
  */
 struct ulist_iterator {
-#ifdef CONFIG_BTRFS_DEBUG
-	int i;
-#endif
 	struct list_head *cur_list;  /* hint to start search */
 };
 
@@ -32,10 +29,6 @@ struct ulist_node {
 	u64 val;		/* value to store */
 	u64 aux;		/* auxiliary value saved along with the val */
 
-#ifdef CONFIG_BTRFS_DEBUG
-	int seqnum;		/* sequence number this node is added */
-#endif
-
 	struct list_head list;  /* used to link node */
 	struct rb_node rb_node;	/* used to speed up search */
 };