Remove pre-grpc driver config validation transport

[unsuman]

I think we can completely remove the pre-grpc stdio pipe transport if we merge FillConfig() and Configure(). It was originally added because we assumed validation would run across all available drivers in the registry, which could slow down things when using gRPC. However, with the new VM-type resolution, we’re guaranteed to either resolve the default driver or use the one explicitly specified by the user.

What do you all think about this?

[AkihiroSuda]

SGTM, we should try to make the API as simple as possible.
We may add back it later if it turned out to be still needed.