OvmfPkg/OvmfXen.dsc: remove PcdCpu* dynamic defaults

OvmfPkg/OvmfXen.dsc: remove PcdCpu* dynamic defaults

PcdCpuMaxLogicalProcessorNumber and PcdCpuApInitTimeOutInMicroSeconds are
only referenced in "OvmfPkg/PlatformPei/PlatformPei.inf", and OvmfXen does
not include that module. Remove the unnecessary dynamic PCD defaults from
"OvmfXen.dsc".

Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Julien Grall <julien.grall@arm.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1515
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Message-Id: <20191022221554.14963-2-lersek@redhat.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>