[MIPS] Remove unused rm9k_cpu_irq_disable()

rm9k_cpu_irq_disable() is unused since commit
1603b5aca4f15b34848fb5594d0c7b6333b99144. Remove it.

Signed-off-by: Atsushi Nemoto <[e-mail]>
Signed-off-by: Ralf Baechle <[e-mail]>

arch/mips/kernel/irq-rm9000.c

···
 	local_irq_restore(flags);
 }
 
-static void rm9k_cpu_irq_disable(unsigned int irq)
-{
-	unsigned long flags;
-
-	local_irq_save(flags);
-	mask_rm9k_irq(irq);
-	local_irq_restore(flags);
-}
-
 /*
  * Performance counter interrupts are global on all processors.
  */