OvmfPkg/PlatformPei: remove Xen support

The "OvmfPkg/PlatformPei/PlatformPei.inf" module is used by the following
platform DSCs:

  OvmfPkg/AmdSev/AmdSevX64.dsc
  OvmfPkg/OvmfPkgIa32.dsc
  OvmfPkg/OvmfPkgIa32X64.dsc
  OvmfPkg/OvmfPkgX64.dsc

Remove Xen support from "OvmfPkg/PlatformPei", including any dependencies
that now become unused. The basic idea is to substitute FALSE for "mXen".

Remove "OvmfPkg/PlatformPei" from the "OvmfPkg: Xen-related modules"
section of "Maintainers.txt".

This patch is best reviewed with "git show -b -W".

Cc: Andrew Fish <afish@apple.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Michael D Kinney <michael.d.kinney@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>
Message-Id: <20210526201446.12554-22-lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>

OvmfPkg/PlatformPei/MemDetect.c

@@ -135,10 +135,6 @@ QemuUc32BaseInitialization (
   UINT32 LowerMemorySize;
   UINT32 Uc32Size;
 
-  if (mXen) {
-    return;
-  }
-
   if (mHostBridgeDevId == INTEL_Q35_MCH_DEVICE_ID) {
     //
     // On q35, the 32-bit area that we'll mark as UC, through variable MTRRs,
@@ -819,11 +815,7 @@ InitializeRamRegions (
   VOID
   )
 {
-  if (!mXen) {
-    QemuInitializeRam ();
-  } else {
-    XenPublishRamRegions ();
-  }
+  QemuInitializeRam ();
 
   if (mS3Supported && mBootMode != BOOT_ON_S3_RESUME) {
     //