UefiCpuPkg/CpuCommonFeaturesLib: Remove XD enable/disable logic
    
    REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2329
    
    XD (ExecutionDisable) feature, when turned on, allows page table
    entry BIT63 set to 1 indicating the memory pointed by the page table
    is disallowed to execute.
    DxeIpl::CreateIdentityMappingPageTables() enables the XD when CPU
    supports it.
    Later DxeCore modifies the page table to set the BIT63 to protect
    the stack/heap to disallow code execution in stack/heap.
    
    UefiCpuPkg/CpuCommonFeaturesLib enables/disables the XD feature
    according to PcdCpuFeaturesSetting.
    When XD is disabled, GP fault is generated immediately because some
    page entries have BIT63 set.
    
    To fix this issue, this patch removes the XD feature logic from
    UefiCpuPkg/CpuCommonFeaturesLib so the XD feature is only taken
    care of by DxeIpl.
    
    Signed-off-by: Ray Ni <ray.ni@intel.com>
    Reviewed-by: Eric Dong <eric.dong@intel.com>
    Acked-by: Laszlo Ersek <lersek@redhat.com>