[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>
diff --git a/drivers/ata/libata-core.c b/drivers/ata/libata-core.c
index 9f87f7d..4f8b8d2 100644
--- a/drivers/ata/libata-core.c
+++ b/drivers/ata/libata-core.c
@@ -5747,7 +5747,8 @@
 
 #ifdef ATA_IRQ_TRAP
 	if ((ap->stats.idle_irq % 1000) == 0) {
-		ap->ops->irq_ack(ap, 0); /* debug trap */
+		ata_chk_status(ap);
+		ap->ops->irq_clear(ap);
 		ata_port_printk(ap, KERN_WARNING, "irq trap\n");
 		return 1;
 	}
@@ -7137,9 +7138,6 @@
 EXPORT_SYMBOL_GPL(ata_eh_qc_retry);
 EXPORT_SYMBOL_GPL(ata_do_eh);
 EXPORT_SYMBOL_GPL(ata_irq_on);
-EXPORT_SYMBOL_GPL(ata_dummy_irq_on);
-EXPORT_SYMBOL_GPL(ata_irq_ack);
-EXPORT_SYMBOL_GPL(ata_dummy_irq_ack);
 EXPORT_SYMBOL_GPL(ata_dev_try_classify);
 
 EXPORT_SYMBOL_GPL(ata_cable_40wire);