[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/pdc_adma.c

@@ -177,7 +177,6 @@ static const struct ata_port_operations adma_ata_ops = {
 	.error_handler		= adma_error_handler,
 	.irq_clear		= adma_irq_clear,
 	.irq_on			= ata_irq_on,
-	.irq_ack		= ata_irq_ack,
 	.port_start		= adma_port_start,
 	.port_stop		= adma_port_stop,
 	.host_stop		= adma_host_stop,