Btrfs: remove rb_node field from the delayed ref node structure

After the last big change in the delayed references code that was needed
for the last qgroups rework, the red black tree node field of struct
btrfs_delayed_ref_node is no longer used, so just remove it, this helps
us save some memory (since struct rb_node is 24 bytes on x86_64) for
these structures.
Signed-off-by: Filipe Manana <fdmanana@suse.com>

fs/btrfs/delayed-ref.h

@@ -34,12 +34,6 @@
  * ref_head. Must clean this mess up later.
  */
 struct btrfs_delayed_ref_node {
-	/*
-	 * ref_head use rb tree, stored in ref_root->href.
-	 * indexed by bytenr
-	 */
-	struct rb_node rb_node;
-
 	/*data/tree ref use list, stored in ref_head->ref_list. */
 	struct list_head list;