UefiCpuPkg: Remove FIT based microcode shadow logic from MpInitLib.

UefiCpuPkg: Remove FIT based microcode shadow logic from MpInitLib.

Commit c7c964b and dd01704 add header file for FIT table and update
MpInitLib to support FIT based microcode shadow operation. There are
comments that FIT is Intel specific specification instead of industry
standard, which should not be placed in EDK2 MdePkg and UefiCpuPkg.
So this patch adds a platform PPI for the microcode shadow logic, and
remove the FIT related code from EDK2.
The FIT based microcode shadow support will be implemented as a new
platform PEIM in IntelSiliconPkg in edk2-platforms.
This patch doesn't provide a DXE version shadow microcode protocol,
a platform which only uses DxeMpInitLib instance only supports PCD
based microcode shadowing.

A detailed design doc can be found here:
https://edk2.groups.io/g/devel/files/Designs/2020/0214/Support%20
the%202nd%20Microcode%20FV%20Flash%20Region.pdf

TEST: Tested on FIT enabled platform.
BZ: https://tianocore.acgmultimedia.com/show_bug.cgi?id=2449

Cc: Eric Dong <eric.dong@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Siyuan Fu <siyuan.fu@intel.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>