Remove unused viewerFont() method

Change-Id: I3da752d7f96162b7cd10e74667a6a3665cb12e14
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/help/litehtmlhelpviewer.h

@@ -45,7 +45,6 @@ public:
     explicit LiteHtmlHelpViewer(QWidget *parent = nullptr);
     ~LiteHtmlHelpViewer() override;
 
-    QFont viewerFont() const override;
     void setViewerFont(const QFont &font) override;
 
     void setScale(qreal scale) override;