renderbackend: removed unusued dialog

This dialog caused a bunch of warnings and was unused in the new renderer
system. It may be used again later, so it's just been commented out, but there's
currently no plan to.

app/render/backend/renderbackend.cpp

@@ -42,7 +42,7 @@ RenderBackend::RenderBackend(QObject *parent) :
   ic_from_conform_(false)
 {
   // FIXME: Don't create in CLI mode
-  cancel_dialog_ = new RenderCancelDialog(Core::instance()->main_window());
+  //cancel_dialog_ = new RenderCancelDialog(Core::instance()->main_window());
 }
 
 RenderBackend::~RenderBackend()