[IA64] Minor cleanups - remove unnecessary function prototype in irq.h

The function prototype for handl_IRQ_event() in include/asm-ia64/irq.h
is no longer needed. This patch removes it.

Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>