x86/io_apic: Remove unused function mp_init_irq_at_boot()

There are no callers in-tree anymore since

  ef9e56d8 ("x86/ioapic: Remove obsolete post hotplug update")

so remove it.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200508140808.49428-1-yuehaibing@huawei.com