MIPS: irq: Avoid an unused-variable error

When CONFIG_IRQ_DOMAIN is set, there is a warning:
arch/mips/kernel/irq.c:114:19: error: unused variable 'desc' [-Werror=unused-variable]
  114 |  struct irq_desc *desc;
      |                   ^~~~

This variable is unused, let's remove it.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211028095652.3503790-1-siyanteng@loongson.cn