[IA64] cleanup dig_irq_init

dig_irq_init is equivalent to machvec_noop, no need to define
another empty function.

Signed-off-by: Ken Chen <kenneth.w.chen@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>

arch/ia64/dig/setup.c

@@ -69,8 +69,3 @@ dig_setup (char **cmdline_p)
 	screen_info.orig_video_isVGA = 1;	/* XXX fake */
 	screen_info.orig_video_ega_bx = 3;	/* XXX fake */
 }
-
-void __init
-dig_irq_init (void)
-{
-}