Btrfs: remove reduplicate check about root in the function btrfs_clean_quota_tree

The check work has been done just before the function btrfs_clean_quota_tree
is called, it is not necessary to check it again, remove it.
Signed-off-by: NWang Shilong <wangsl-fnst@cn.fujitsu.com>
Signed-off-by: NJosef Bacik <jbacik@fusionio.com>

fs/btrfs/qgroup.c

@@ -742,9 +742,6 @@ static int btrfs_clean_quota_tree(struct btrfs_trans_handle *trans,
 	int ret;
 	int nr = 0;
 
-	if (!root)
-		return -EINVAL;
-
 	path = btrfs_alloc_path();
 	if (!path)
 		return -ENOMEM;