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>

arch/mips/include/asm/irq.h

@@ -63,8 +63,6 @@ extern void do_domain_IRQ(struct irq_domain *domain, unsigned int irq);
 extern void arch_init_irq(void);
 extern void spurious_interrupt(void);
 
-extern int allocate_irqno(void);
-
 /*
  * Before R2 the timer and performance counter interrupts were both fixed to
  * IE7.	 Since R2 their number has to be read from the c0_intctl register.