KVM: Remove kvm_arch_vcpu_free() declaration

Remove KVM's declaration of kvm_arch_vcpu_free() now that the function
is gone from all architectures (several architectures were relying on
the forward declaration).

Acked-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>