libata: remove libata.spindown_compat

With STANDBYDOWN tracking added, libata.spindown_compat isn't
necessary anymore.  If userspace shutdown(8) issues STANDBYNOW, libata
warns.  If userspace shutdown(8) doesn't issue STANDBYNOW, libata does
the right thing.  Userspace can tell whether kernel supports spindown
by testing whether sysfs node manage_start_stop exists as before.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>