KVM: arm/arm64: vgic-its: Remove kvm_its_unmap_device

Let's remove kvm_its_unmap_device and use kvm_its_free_device
as both functions are identical.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>