MIPS: IRQ: remove orphan allocate_irqno() declaration

All uses of allocate_irqno() have been removed by
commit 69a07a41

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

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>