powerpc: Remove i8259 irq_host_ops->unmap

It was never called because the host is always IRQ_HOST_MAP_LEGACY.

And what it purported to do was mask the interrupt (which will already
have happend if we shutdown the interrupt), then synchronise_irq and
clear the chip pointer, both of which will have been be done by the
caller were we to call unmap on a legacy irq.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>