OvmfPkg: remove the Xen drivers from the IA32, IA32X64, and X64 platforms
    
    Remove the three Xen drivers as the first step for removing Xen support
    from the historical OvmfPkg* platforms. Xen (HVM and PVH) guests are
    supported by the dedicated OvmfXen platform.
    
    No module remains dependent on XenHypercallLib, so remove the
    XenHypercallLib class resolutions too, from the DSC files.
    
    Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
    Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2122
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>