Remove dead code related to compositing from Sky

sky/viewer/platform/platform_impl.cc

Index: sky/viewer/platform/platform_impl.cc
diff --git a/sky/viewer/platform/platform_impl.cc b/sky/viewer/platform/platform_impl.cc
index 7766292fc6d6f2b71eaffd1bcd6a05b5a26b85af..ace59a38379dcbfe7c96c930ec7f63443c6bd451 100644
--- a/sky/viewer/platform/platform_impl.cc
+++ b/sky/viewer/platform/platform_impl.cc
@@ -110,10 +110,6 @@ base::SingleThreadTaskRunner* PlatformImpl::mainThreadTaskRunner() {
   return main_thread_task_runner_.get();
 }
 
-bool PlatformImpl::isThreadedCompositingEnabled() {
-  return true;
-}
-
 mojo::NetworkService* PlatformImpl::networkService() {
   return network_service_.get();
 }