[IA64] fix section mismatch in arch/ia64/kernel/irq.c

This patch shuts up the following:

	WARNING: vmlinux.o(.text+0x7102): Section mismatch in
	reference from the function fixup_irqs() to the function
	.devinit.text:ia64_disable_timer()

Removing ia64_disable_timer() is safe because there are no functions
calling it other than the fixup_irqs(),

Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>