pc: Drop useless test from isa_irq_handler
IsaIrqState::ioapic is always non-NULL. Probably, the concrete
qemu_irq was supposed to be tested, but that's already done by
qemu_set_irq.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>