KVM: arm/arm64: vgic: Drop the kvm_vgic_register_mmio_region()

kvm_vgic_register_mmio_region() was introduced in commit 4493b1c4


("KVM: arm/arm64: vgic-new: Add MMIO handling framework") but never
used, and even never implemented. Remove it to avoid confusing readers.

Reported-by: Haibin Wang <wanghaibin.wang@huawei.com>
Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200119090604.398-1-yuzenghui@huawei.com