btrfs: cleanup the useless DEFINE_WAIT in cleanup_transaction

When it was introduced in commit f094ac32 ("Btrfs: fix NULL pointer
after aborting a transaction"), it was not used.
Signed-off-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/transaction.c

@@ -1840,7 +1840,6 @@ static void cleanup_transaction(struct btrfs_trans_handle *trans, int err)
 {
 	struct btrfs_fs_info *fs_info = trans->fs_info;
 	struct btrfs_transaction *cur_trans = trans->transaction;
-	DEFINE_WAIT(wait);
 
 	WARN_ON(refcount_read(&trans->use_count) > 1);