Remove deprecated methods from FlutterViewController (flutter#21081)

The template now uses the FlutterEngine versions.

shell/platform/windows/client_wrapper/flutter_view_controller.cc

@@ -40,13 +40,4 @@ std::optional<LRESULT> FlutterViewController::HandleTopLevelWindowProc(
   return handled ? result : std::optional<LRESULT>(std::nullopt);
 }
 
-std::chrono::nanoseconds FlutterViewController::ProcessMessages() {
-  return engine_->ProcessMessages();
-}
-
-FlutterDesktopPluginRegistrarRef FlutterViewController::GetRegistrarForPlugin(
-    const std::string& plugin_name) {
-  return engine_->GetRegistrarForPlugin(plugin_name);
-}
-
 }  // namespace flutter

shell/platform/windows/client_wrapper/include/flutter/flutter_view_controller.h

@@ -24,7 +24,7 @@ namespace flutter {
 // This is the primary wrapper class for the desktop C API.
 // If you use this class, you should not call any of the setup or teardown
 // methods in the C API directly, as this class will do that internally.
-class FlutterViewController : public PluginRegistry {
+class FlutterViewController {
  public:
   // Creates a FlutterView that can be parented into a Windows View hierarchy
   // either using HWNDs or in the future into a CoreWindow, or using compositor.
@@ -56,13 +56,6 @@ class FlutterViewController : public PluginRegistry {
                                                   WPARAM wparam,
                                                   LPARAM lparam);
 
-  // DEPRECATED. Call engine()->ProcessMessages() instead.
-  std::chrono::nanoseconds ProcessMessages();
-
-  // DEPRECATED. Call engine()->GetRegistrarForPlugin() instead.
-  FlutterDesktopPluginRegistrarRef GetRegistrarForPlugin(
-      const std::string& plugin_name) override;
-
  private:
   // Handle for interacting with the C API's view controller, if any.
   FlutterDesktopViewControllerRef controller_ = nullptr;