Use cpu_has_vmx_rdtscp() directly when computing secondary exec controls
and drop the now defunct vmx_rdtscp_supported().

No functional change intended.
Reviewed-by: NVitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: NSean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>