gRPC callouts: remove specialized hostcalls

[PiotrSikora]

Since gRPC it's built on top of HTTP, it's debatable whether we need a specialized hostcall for gRPC callouts or if SDKs could provide them themselves.

As far as I recall, we've added those because Envoy implemented them using grpc library that automatically authenticated service accounts on GCP, but I'm not sure if that's still needed? cc @kyessenov