OvmfPkg: remove the Xen drivers from the AmdSev platform

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>
Message-Id: <20210526201446.12554-3-lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>