Btrfs: remove reduplicate check when disabling quota

We have checked 'quota_root' with qgroup_ioctl_lock held before,So
here the check is reduplicate, remove it.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Reviewed-by: Miao Xie <miaox@cn.fujitsu.com>
Reviewed-by: Arne Jansen <sensille@gmx.net>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/qgroup.c

@@ -943,11 +943,6 @@ int btrfs_quota_disable(struct btrfs_trans_handle *trans,
 
 	btrfs_free_qgroup_config(fs_info);
 
-	if (!quota_root) {
-		ret = -EINVAL;
-		goto out;
-	}
-
 	ret = btrfs_clean_quota_tree(trans, quota_root);
 	if (ret)
 		goto out;