MIPS: IRQ: remove orphan allocate_irqno() declaration

All uses of allocate_irqno() have been removed by
commit 69a07a41d908 ("MIPS: SGI-IP27: rework HUB interrupts"),
so remove the orphan declaration.

Signed-off-by: Gaosheng Cui <[e-mail]>
Reviewed-by: Philippe Mathieu-Daudé <[e-mail]>
Signed-off-by: Thomas Bogendoerfer <[e-mail]>

arch/mips/include/asm/irq.h

···
 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.