Don't use separate threads for querying SDL video info

This is no longer necessary now that we force Qt and SDL to use EGL.

app/backend/systemproperties.h

@@ -38,8 +38,6 @@ signals:
 
 private:
     void querySdlVideoInfo();
-    void querySdlVideoInfoInternal();
-    void refreshDisplaysInternal();
 
     bool hasHardwareAcceleration;
     bool rendererAlwaysFullScreen;