xfs_scrub: remove preen mode

xfs_scrub: remove preen mode

While it's true that the kernel can tell us whether something needs
repairs or it needs optimizing, from the admin's perspective there's
no point in having an optimize-only mode -- either fix everything, or
don't.  This is what xfs_repair does w.r.t. -n, so let's do the same
thing too.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>