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>