MIPS: Loongson: Remove unused macro LOONGSON_PERFCNT_IRQ

LOONGSON2_PERFCNT_IRQ is used for the irq number of the performance
overflow interrupts; LOONGSON_PERFCNT_IRQ is unused so remove it.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1494/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>