irqchip/apple-aic: Drop unused ipi_hwirq field

This field was never used, remove it.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Hector Martin <marcan@marcan.st>
Link: https://lore.kernel.org/r/20220108140118.3378937-1-maz@kernel.org