irqchip/mips: Drop selection of IRQ_DOMAIN_HIERARCHY

Now that GENERIC_IRQ_IPI selects IRQ_DOMAIN_HIERARCHY, there is no
need to have this conditional select for IRQ_MIPS_CPU. Similarily,
MIPS_GIC only needs selecting GENERIC_IRQ_IPI.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Marc Zyngier <maz@kernel.org>