libata: remove spindown skipping and warning

This was a hack to give userland shutdown tools time to drop manual
spindown.  All popular distros updated quite some time ago and the due
is well passed.  Drop it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jaswinder Singh Rajput <jaswinder@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>