Mojo: Only link against one system impl

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',
@@ skipping 373 matching lines @@
     },
     {
       'target_name': 'mojo_surfaces_service',
       'type': 'loadable_module',
       'dependencies': [
         '../base/base.gyp:base',
         '../cc/cc.gyp:cc',
         '../cc/cc.gyp:cc_surfaces',
         '../ui/gfx/gfx.gyp:gfx_geometry',
         'mojo_base.gyp:mojo_environment_chromium',
-        'mojo_base.gyp:mojo_system_impl',
         'mojo_base.gyp:mojo_application_chromium',
         'mojo_cc_support',
         'mojo_geometry_bindings',
         'mojo_geometry_lib',
         'mojo_surfaces_bindings',
         'mojo_surfaces_lib',
         '<(mojo_gles2_for_loadable_module)',
         '<(mojo_system_for_loadable_module)',
       ],
       'sources': [
@@ skipping 453 matching lines @@
             'mojo_echo_bindings',
           ],
           'sources': [
             'services/dbus_echo/dbus_echo_service.cc',
           ],
         },
       ],
     }],
   ],
 }