irqchip/irq-bcm7038-l1: Remove .irq_cpu_offline()

With arch/mips/kernel/smp-bmips.c having been migrated away from
irq_cpu_offline() and use irq_migrate_all_off_this_cpu() instead, we no
longer need to implement an .irq_cpu_offline() callback. This is a
necessary change to facilitate the building of this driver as a module.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211020184859.2705451-3-f.fainelli@gmail.com