OvmfPkg/SmmCpuFeaturesLib: drop obsolete API implementation
    
    Commit 0426115b6738 ("UefiCpuPkg: Remove unused API in
    SmmCpuFeaturesLib.h", 2022-12-21) removed the declaration of the function
    SmmCpuFeaturesAllocatePageTableMemory() from the "SmmCpuFeaturesLib.h"
    library class header.
    
    Remove the API's (null-)implementation from OvmfPkg/SmmCpuFeaturesLib as
    well.
    
    Testing: OVMF builds, boots, and suspends/resumes (see earlier in this
    series).
    
    Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
    Cc: Gerd Hoffmann <kraxel@redhat.com>
    Cc: Jiewen Yao <jiewen.yao@intel.com>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Bugzilla: https://bugzilla.tianocore.org/show_bug.cgi?id=4235
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>