remove timestamp from walkthrough example

[raboof]

We recently made it easier to pull in the standard dependencies, and added a timestamp to the 'helloworld' proto to test that.

Unfortunately, that was the .proto we also use for the walkthrough in the documentation (https://doc.akka.io/docs/akka-grpc/snapshot/client/walkthrough.html). I think for the walkthrough we should keep things as simple as possible, so make sure we use a proto without external types.

[ignasi35]

Did a first hack in #1260 (which hides the unnecessary complexity from the rendered HTML).

The long-term solution should be:

• create a new proto file
• remove the use of external dependencies from ./plugin-tester-scala/src/main/protobuf/helloworld.proto (loosing the scala version of the "use external dependencies test")
• ... (?)