btrfs: remove unused is_head field from struct btrfs_delayed_ref_node

The 'is_head' field of struct btrfs_delayed_ref_node is no longer after
commit d278850e ("btrfs: remove delayed_ref_node from ref_head"),
so remove it.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/delayed-ref.c

@@ -853,7 +853,6 @@ static void init_delayed_ref_common(struct btrfs_fs_info *fs_info,
 	ref->num_bytes = num_bytes;
 	ref->ref_mod = 1;
 	ref->action = action;
-	ref->is_head = 0;
 	ref->in_tree = 1;
 	ref->seq = seq;
 	ref->type = ref_type;

fs/btrfs/delayed-ref.h

@@ -48,8 +48,6 @@ struct btrfs_delayed_ref_node {
 
 	unsigned int action:8;
 	unsigned int type:8;
-	/* is this node still in the rbtree? */
-	unsigned int is_head:1;
 	unsigned int in_tree:1;
 };