x86: remove dead IRQBALANCE code

Impact: cleanup

CONFIG_IRQBALANCE was removed in commit 8b8e8c1b; this ifdef was still
around.
Signed-off-by: NDan McGee <dpmcgee@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/include/asm/irq.h

@@ -31,10 +31,6 @@ static inline int irq_canonicalize(int irq)
 # endif
 #endif
 
-#ifdef CONFIG_IRQBALANCE
-extern int irqbalance_disable(char *str);
-#endif
-
 #ifdef CONFIG_HOTPLUG_CPU
 #include <linux/cpumask.h>
 extern void fixup_irqs(cpumask_t map);