KVM: arm64: Delete pointless switch statement in kvm_reset_vcpu()

The vCPU target hasn't mattered for quite a long time now. Delete the
useless switch statement in kvm_reset_vcpu(), which hilariously only had
a default case in it.

Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230710193140.1706399-2-oliver.upton@linux.dev