OvmfPkg: remove the Xen drivers from the AmdSev platform
    
    For symmetry with the historical OvmfPkg* platforms, remove the three Xen
    drivers from the remotely attested, QEMU/KVM-only, AmdSev platform. Xen
    (HVM and PVH) guests are supported by the dedicated OvmfXen platform.
    
    No module remains dependent on XenHypercallLib, so remove the
    XenHypercallLib class resolution too, from the DSC file.
    
    Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
    Cc: Brijesh Singh <brijesh.singh@amd.com>
    Cc: Erdem Aktas <erdemaktas@google.com>
    Cc: James Bottomley <jejb@linux.ibm.com>
    Cc: Jiewen Yao <jiewen.yao@intel.com>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Cc: Min Xu <min.m.xu@intel.com>
    Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
    Cc: Tom Lendacky <thomas.lendacky@amd.com>
    Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2122
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>