genirq/irq_sim: Remove unused irq_sim_work_ctx:: Irq_base

Since commit 337cbeb2 ("genirq/irq_sim: Simplify the API"),
irq_sim_work_ctx::irq_base is unused. Drop it.

Found by https://github.com/jirislaby/clang-struct

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Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Link: https://lore.kernel.org/all/20240808104118.430670-1-jirislaby@kernel.org