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/settingsdialog.h

@@ -23,7 +23,6 @@ public slots:
 
 private:
     QCheckBox * m_stayOnTop = nullptr;
-    QCheckBox * m_useBuiltInCloseAnimation = nullptr;
     QCheckBox * m_useLightCheckerboard = nullptr;
     QCheckBox * m_loopGallery = nullptr;
     QCheckBox * m_svgTiny12Only = nullptr;