MIPS: IRQ: remove orphan declarations from arch/mips/include/asm/irq.h

All uses of free_irqno() and alloc_legacy_irqno() were removed
by commit 69a07a41

 ("MIPS: SGI-IP27: rework HUB interrupts"),
so remove the orphan declarations, too.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>