powerpc/irq: drop arch_early_irq_init()

arch_early_irq_init() does nothing different than the weak
arch_early_irq_init() in kernel/softirq.c

Fixes: 089fb442 ("powerpc: Use ARCH_IRQ_INIT_FLAGS")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>