genirq: Remove unused extern declaration

commit 3795de23 ("genirq: Distangle kernel/irq/handle.c")
left behind this.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230720143625.29176-1-yuehaibing@huawei.com