btrfs: remove unused member of btrfs_trans_handle

Last user was removed in a monster commit a22285a6
("Btrfs: Integrate metadata reservation with start_transaction") in
2010.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/transaction.h

@@ -112,7 +112,6 @@ struct btrfs_trans_handle {
 	u64 bytes_reserved;
 	u64 chunk_bytes_reserved;
 	unsigned long use_count;
-	unsigned long blocks_reserved;
 	unsigned long delayed_ref_updates;
 	struct btrfs_transaction *transaction;
 	struct btrfs_block_rsv *block_rsv;