Help: Remove unused method and related code

Change-Id: I4c4641290eceb441339cc237ee74a52c1018e3ae
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/help/macwebkithelpviewer.h

@@ -81,7 +81,6 @@ public:
     QUrl source() const override;
     void setSource(const QUrl &url) override;
     void scrollToAnchor(const QString &anchor);
-    void highlightId(const QString &id) override { Q_UNUSED(id) }
 
     void setHtml(const QString &html) override;