Mojo: Remove unused hello_world_service code

mojo/examples/hello_world_service/hello_world_service_impl.cc

-// Copyright 2013 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#include "mojo/examples/hello_world_service/hello_world_service_impl.h"
-
-#include <string>
-
-#include "base/logging.h"
-
-namespace mojo {
-namespace examples {
-
-HelloWorldServiceImpl::HelloWorldServiceImpl(ScopedMessagePipeHandle pipe)
-    : client_(pipe.Pass()) {
-  client_.SetPeer(this);
-}
-
-void HelloWorldServiceImpl::Greeting(const String& greeting) {
-  LOG(INFO) << greeting.To<std::string>();
-  client_->DidReceiveGreeting(42);
-}
-
-HelloWorldServiceImpl::~HelloWorldServiceImpl() {}
-
-}  // namespace examples
-}  // namespace mojo