Remove redundant echo.mojom, verify non-nullables in echo_service.mojom

mojo/services/BUILD.gn

 # Copyright 2014 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.
 
 import("//build/config/ui.gni")
 
 group("services") {
   deps = [
-    "//mojo/services/dbus_echo:bindings",
     "//mojo/services/gles2:bindings",
     "//mojo/services/network",
     "//mojo/services/public/interfaces/content_handler",
     "//mojo/services/public/interfaces/geometry",
     "//mojo/services/public/interfaces/input_events",
     "//mojo/services/public/interfaces/launcher",
     "//mojo/services/public/interfaces/native_viewport",
     "//mojo/services/public/interfaces/navigation",
     "//mojo/services/public/interfaces/network",
     "//mojo/services/public/interfaces/surfaces",
     "//mojo/services/surfaces",
     "//mojo/services/test_service:bindings",
   ]
   if (use_aura) {
     deps += [
       "//mojo/services/public/interfaces/view_manager",
       "//mojo/services/public/interfaces/window_manager",
       "//mojo/services/window_manager",
     ]
   }
 }