sh: Remove unused declarations for make_maskreg_irq() and irq_mask_register

make_maskreg_irq() and irq_mask_register have been removed since
commit 5a4053b2 ("sh: Kill off dead boards."), so remove the
unused declarations.
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>

arch/sh/include/asm/irq.h

@@ -13,12 +13,6 @@
  */
 #define NO_IRQ_IGNORE		((unsigned int)-1)
 
-/*
- * Simple Mask Register Support
- */
-extern void make_maskreg_irq(unsigned int irq);
-extern unsigned short *irq_mask_register;
-
 /*
  * PINT IRQs
  */