iommu/irq_remapping: Kill most of hyperv-iommu.c now it's redundant

iommu/irq_remapping: Kill most of hyperv-iommu.c now it's redundant

It took me a while to realise that this "IRQ remapping" driver exists
not to actually remap interrupts, but to return -EINVAL if anyone ever
tries to set the affinity to a set of CPUs which can't be reached
*without* remapping. Having fixed the core IRQ domain code to handle
such limited, all this hackery can now die.

I haven't deleted it entirely because its existence still causes the
kernel to use X2APIC in cluster mode not physical. I'm not sure we
*want* that, but it wants further thought and testing before ripping
it out too.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>