btrfs: drop useless member qgroup_reserved of btrfs_pending_snapshot

Since there is no more use of qgroup_reserved member in struct
btrfs_pending_snapshot, remove it.
Signed-off-by: NGu JinXiang <gujx@cn.fujitsu.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/transaction.h

@@ -139,7 +139,6 @@ struct btrfs_pending_snapshot {
 	struct btrfs_path *path;
 	/* block reservation for the operation */
 	struct btrfs_block_rsv block_rsv;
-	u64 qgroup_reserved;
 	/* extra metadata reservation for relocation */
 	int error;
 	bool readonly;