UefiCpuPkg/PiSmmCpuDxeSmm: drop support for obsolete processors

It's highly unlikely the code ever runs on processors which are
almost 30 years old.  Drop the code handling them.

Bugzilla: https://bugzilla.tianocore.org/show_bug.cgi?id=4345


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>