[libata] Remove ->irq_ack() hook, and ata_dummy_irq_on()

* ->irq_ack() is redundant to what the irq handler already
  performs... chk-status + irq-clear.  Furthermore, it is only
  called in one place, when screaming-irq-debugging is enabled,
  so we don't want to bother with a hook just for that.

* ata_dummy_irq_on() is only ever used in drivers that have
  no callpath reaching ->irq_on().  Remove .irq_on hook from
  those drivers, and the now-unused ata_dummy_irq_on()

Signed-off-by: Jeff Garzik <jeff@garzik.org>

drivers/ata/pata_oldpiix.c

@@ -266,7 +266,6 @@ static const struct ata_port_operations oldpiix_pata_ops = {
 	.irq_handler		= ata_interrupt,
 	.irq_clear		= ata_bmdma_irq_clear,
 	.irq_on			= ata_irq_on,
-	.irq_ack		= ata_irq_ack,
 
 	.port_start		= ata_port_start,
 };