revert: remove the ability that disable built-in close window animation

no need to preserve this option because frameless window and
close window animation will be fully removed in future.

app/mainwindow.h

@@ -106,8 +106,6 @@ private slots:
     void on_actionLocateInFileManager_triggered();
     void on_actionQuitApp_triggered();
 
-    void doCloseWindow();
-
 private:
     bool updateFileWatcher(const QString & basePath = QString());
     void updateGalleryButtonsVisibility();