libata: remove ATA_FLAG_NO_LEGACY

All checks of ATA_FLAG_NO_LEGACY have been removed by the commits
c791c306 ([libata] minor PCI IDE probe
fixes and cleanups) and f0d36efd (libata:
update libata core layer to use devres), so I think it's time to finally
get rid of this flag...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>