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

mojo/mojo_services.gypi

 # Copyright (c) 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.
 
 {
   'targets': [
     {
-      # GN version: //mojo/services/dbus_echo:bindings
-      'target_name': 'mojo_echo_bindings',
-      'type': 'static_library',
-      'sources': [
-        'services/dbus_echo/echo.mojom',
-      ],
-      'includes': [ 'public/tools/bindings/mojom_bindings_generator.gypi' ],
-      'export_dependent_settings': [
-        'mojo_base.gyp:mojo_cpp_bindings',
-      ],
-      'dependencies': [
-        'mojo_base.gyp:mojo_cpp_bindings',
-      ],
-    },
-    {
       'target_name': 'mojo_html_viewer',
       'type': 'loadable_module',
       'dependencies': [
         '../net/net.gyp:net',
         '../skia/skia.gyp:skia',
         '../third_party/WebKit/public/blink.gyp:blink',
         '../ui/native_theme/native_theme.gyp:native_theme',
         '../url/url.gyp:url_lib',
         'mojo_base.gyp:mojo_common_lib',
         'mojo_base.gyp:mojo_cpp_bindings',
@@ skipping 842 matching lines @@
             }],
             ['use_x11==1', {
               'dependencies': [
                 '../ui/gfx/x/gfx_x11.gyp:gfx_x11',
               ],
             }],
           ],
         },
       ],
     }],
-    ['OS=="linux"', {
-      'targets': [
-        {
-          'target_name': 'mojo_dbus_echo_service',
-          'type': 'executable',
-          'dependencies': [
-            '../base/base.gyp:base',
-            '../build/linux/system.gyp:dbus',
-            '../dbus/dbus.gyp:dbus',
-            'mojo_base.gyp:mojo_common_lib',
-            'mojo_base.gyp:mojo_system_impl',
-            'mojo_base.gyp:mojo_application_chromium',
-            'mojo_dbus_service',
-            'mojo_echo_bindings',
-          ],
-          'sources': [
-            'services/dbus_echo/dbus_echo_service.cc',
-          ],
-        },
-      ],
-    }],
   ],
 }