Use simpler Plugin::initialize() when feasible

Change-Id: I567965d266f20526bda9f823e31a04b354d53fb1
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/classview/classviewplugin.h

@@ -18,7 +18,7 @@ public:
     ~ClassViewPlugin() final;
 
 private:
-    bool initialize(const QStringList &arguments, QString *errorMessage = nullptr) final;
+    void initialize() final;
     void extensionsInitialized() final {}
 };