UefiCpuPkg/SmmCommunication: Remove out-dated comments

UefiCpuPkg/SmmCommunication: Remove out-dated comments

The comments in PiSmmCommunicationPei.c describe the whole memory
layout of the SMRAM regarding the SMM communication.

But SHA-1: 8b1d14939053b63d80355465649c50f9f391a64a
PiSmmCommunicationSmm: Deprecate SMM Communication ACPI Table
removed the code that produces the ACPI Table.

This change updates the accordingly comments.

Signed-off-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>