xfs: remove deprecated sysctls

xfsbufd_centisecs and age_buffer_centisecs were due for removal in
3.14. We forgot to do that - it's now well past time to remove these
deprecated, unused sysctls.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>