KVM: x86: Drop kvm_arch_vcpu_free()

Remove the superfluous kvm_arch_vcpu_free() as it is no longer called
from commmon KVM code.  Note, kvm_arch_vcpu_destroy() *is* called from
common code, i.e. choosing which function to whack is not completely
arbitrary.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>