genirq/irq_sim: Remove unused field from struct irq_sim_irq_ctx

The irqnum field is unused. Remove it.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240122124243.44002-3-brgl@bgdev.pl