Help: Remove unused method and related code

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

src/plugins/help/helpviewer.h

@@ -62,7 +62,6 @@ public:
     virtual QUrl source() const = 0;
     // metacall in HelpPlugin::updateSideBarSource
     Q_INVOKABLE virtual void setSource(const QUrl &url) = 0;
-    virtual void highlightId(const QString &id) { Q_UNUSED(id) }
 
     virtual void setHtml(const QString &html) = 0;