Use simpler Plugin::initialize() when feasible

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

src/plugins/autotest/autotestplugin.h

@@ -39,7 +39,7 @@ public:
     AutotestPlugin();
     ~AutotestPlugin() override;
 
-    bool initialize(const QStringList &arguments, QString *errorString) override;
+    void initialize() override;
     void extensionsInitialized() override;
     ShutdownFlag aboutToShutdown() override;