btrfs: remove junk sb_dirt change

BTRFS does not define a '->write_super()' method, so it should
not mark its superblock as dirty. This looks like some left-over.
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Acked-by: Chris Mason <chris.mason@oracle.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/btrfs/inode.c

@@ -2938,7 +2938,6 @@ int btrfs_unlink_subvol(struct btrfs_trans_handle *trans,
 	dir->i_mtime = dir->i_ctime = CURRENT_TIME;
 	ret = btrfs_update_inode(trans, root, dir);
 	BUG_ON(ret);
-	dir->i_sb->s_dirt = 1;
 
 	btrfs_free_path(path);
 	return 0;