btrfs: remove unused member list from btrfs_end_io_wq

The end io work queue items have been tracked by the work queues since
"Btrfs: Add async worker threads for pre and post IO checksumming"
(8b712842) (2008).
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -89,7 +89,6 @@ struct btrfs_end_io_wq {
 	struct btrfs_fs_info *info;
 	int error;
 	enum btrfs_wq_endio_type metadata;
-	struct list_head list;
 	struct btrfs_work work;
 };