Remove //sky/engine/platform/network

This code is all dead. Instead, we use Mojo directly from Dart to access the
network.

TBR=eseidel@google.com

Review URL: https://codereview.chromium.org/1239633002 .

engine/src/flutter/shell/ui/engine.cc

@@ -80,7 +80,7 @@ void Engine::Init(ServiceProviderContext* service_provider_context) {
   mojo::ConnectToService(service_provider.get(), &network_service);
 
   DCHECK(!g_platform_impl);
-  g_platform_impl = new PlatformImpl(network_service.Pass());
+  g_platform_impl = new PlatformImpl();
   blink::initialize(g_platform_impl);
 }