OvmfPkg: remove CSM_ENABLE build macro

At this point, the CSM_ENABLE conditionals only bracket the !error
directives that we added at the front of this series; it's time to remove
CSM_ENABLE.

Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Corvin Köhne <corvink@freebsd.org>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Rebecca Cran <rebecca@bsdio.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=4588
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20231110235820.644381-38-lersek@redhat.com>
Reviewed-by: Jiewen Yao <Jiewen.yao@intel.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Corvin Köhne <corvink@FreeBSD.org>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>

OvmfPkg/OvmfXen.dsc

@@ -363,9 +363,6 @@
   gEfiMdeModulePkgTokenSpaceGuid.PcdConOutGopSupport|TRUE
   gEfiMdeModulePkgTokenSpaceGuid.PcdConOutUgaSupport|FALSE
   gEfiMdeModulePkgTokenSpaceGuid.PcdInstallAcpiSdtProtocol|TRUE
-!ifdef $(CSM_ENABLE)
-!error "CSM is being torn down"
-!endif
 
 [PcdsFixedAtBuild]
   gEfiMdeModulePkgTokenSpaceGuid.PcdPciDisableBusEnumeration|TRUE