Delete Step_3.md

Remove separate step now that all the content has been moved into the main doc

Step_3.md

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-# Step-3: Implement a server.
-
-This step extends the generated server skeleton code to write a simple server
-that provides the hello service. This introduces two new classes:
-
-- a service implementation [GreetingsImpl.java](src/main/java/ex/grpc/GreetingsImpl.java).
-
-- a server that hosts the service implementation and allows access over the network: [GreetingsServer.java](src/main/java/ex/grpc/GreetingsServer.java).
-
-## Service implementation
-
-[GreetingsImpl.java](src/main/java/ex/grpc/GreetingsImpl.java)
-implements the behaviour we require of our GreetingService. There are a
-number of important features of gRPC being used here:
-
-```
-  public void hello(Helloworld.HelloRequest req,
-      StreamObserver<Helloworld.HelloReply> responseObserver) {
-    Helloworld.HelloReply reply = Helloworld.HelloReply.newBuilder().setMessage(
-        "Hello " + req.getName()).build();
-    responseObserver.onValue(reply);
-    responseObserver.onCompleted();
-  }
-```
-
-- it provides a class `GreetingsImpl` that implements a generated interface `GreetingsGrpc.Greetings`
-- `GreetingsGrpc.Greetings` declares the method `hello` that was declared in the proto [IDL](src/main/proto/helloworld.proto)
-- `hello's` signature is typesafe:
-   hello(Helloworld.HelloRequest req, StreamObserver<Helloworld.HelloReply> responseObserver)
-- `hello` takes two parameters:
-  `Helloworld.HelloRequest`: the request
-  `StreamObserver<Helloworld.HelloReply>`: a response observer, an interface to be called with the response value
-- to complete the call
-  - the return value is constructed
-  - the responseObserver.onValue() is called with the response
-  - responseObserver.onCompleted() is called to indicate that no more work will done on the RPC.
-
-
-## Server implementation
-
-[GreetingsServer.java](src/main/java/ex/grpc/GreetingsServer.java) shows the
-other main feature required to provde the gRPC service; how to allow a service
-implementation to be accessed from the network.
-
-```
-  private void start() throws Exception {
-    server = NettyServerBuilder.forPort(port)
-             .addService(GreetingsGrpc.bindService(new GreetingsImpl()))
-             .build();
-    server.startAsync();
-    server.awaitRunning(5, TimeUnit.SECONDS);
-  }
-
-```
-
-- it provides a class `GreetingsServer` that holds a `ServerImpl` that will run the server
-- in the `start` method, `GreetingServer` binds the `GreetingsService` implementation to a port and begins running it
-- there is also a `stop` method that takes care of shutting down the service and cleaning up when the program exits
-
-## Build it
-
-This is the same as before: our client and server are part of the same maven
-package so the same command builds both.
-
-```
-$ mvn package
-```
-
-## Try them out
-
-We've added simple shell scripts to simplifying running the examples. Now
-that they are built, you can run the server with:
-
-```
-$ ./run_greetings_server.sh
-```
-
-and in another terminal window confirm that it receives a message.
-
-```
-$ ./run_greetings_client.sh
-```