ArmVirtPkg; avoid DxeTpmMeasurementLib in shared .DSC

ArmVirtPkg; avoid DxeTpmMeasurementLib in shared .DSC

DxeTpmMeasurementLib should only be used on platforms that implement
measured boot, which we will do in a future patch, but only for
ArmVirtQemu, as the remaining ones are fundamentally incompatible,
given that they do not implement a PEI phase.

So use TpmMeasurementLibNull as the default resolution for all
ArmVirtPkg platforms, regardless of how they are built.

This mirrors commit 1ec05b81e59f ("OvmfPkg: use DxeTpmMeasurementLib
if and only if TPM2_ENABLE", 2019-07-04).

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2560
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>