powerpc/hwirq: Remove stale forward irq_chip declaration

Since commit identified below, the forward declaration of
struct irq_chip is useless (was struct hw_interrupt_type at that time)

Remove it, together with the associated comment.

Fixes: c0ad90a3 ("[PATCH] genirq: add ->retrigger() irq op to consolidate hw_irq_resend()")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/fbe58d27cf128d5fe581e4510ded8701858f268e.1596716328.git.christophe.leroy@csgroup.eu