CVE-2017-5753 (Spectre v1 Intel)

With the switch to using LFENCE_RDTSC on AMD platforms there is no longer
a need for the MFENCE_RDTSC feature.  Remove its usage and definition.
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>