[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_ali.c

@@ -327,7 +327,6 @@ static struct ata_port_operations ali_early_port_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,
 };
@@ -369,7 +368,6 @@ static struct ata_port_operations ali_20_port_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,
 };
@@ -408,7 +406,6 @@ static struct ata_port_operations ali_c2_port_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,
 };
@@ -446,7 +443,6 @@ static struct ata_port_operations ali_c5_port_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,
 };