arm/arm64: KVM: Remove external abort test from MMIO handling

As we know handle external aborts pretty early, we can get rid of
its handling in the MMIO code (which was a bit odd to begin with...).

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>