Mojo: Only link against one system impl

In the static build mojo_surfaces_service and some examples were linking
against both mojo_system and mojo_system_impl, which doesn't work.

R=erg@chromium.org

Review URL: https://codereview.chromium.org/468563002

Cr-Commit-Position: refs/heads/master@{#289134}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289134 0039d316-1c4b-4281-b951-d872f2087c98

mojo/mojo_examples.gypi

@@ -372,7 +372,6 @@
         'mojo_base.gyp:mojo_application_chromium',
         'mojo_base.gyp:mojo_common_lib',
         'mojo_base.gyp:mojo_environment_chromium',
-        'mojo_base.gyp:mojo_system_impl',
         'mojo_geometry_bindings',
         'mojo_geometry_lib',
         'mojo_native_viewport_bindings',
@@ -426,7 +425,6 @@
         'mojo_base.gyp:mojo_application_chromium',
         'mojo_base.gyp:mojo_common_lib',
         'mojo_base.gyp:mojo_environment_chromium',
-        'mojo_base.gyp:mojo_system_impl',
         'mojo_geometry_bindings',
         'mojo_geometry_lib',
         'mojo_surfaces_app_bindings',

mojo/mojo_services.gypi

@@ -391,7 +391,6 @@
         '../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',