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>