Core: Simplify MainWindow::init() signature

And adapt its only user.

Change-Id: Ifc1f6c7857951d6e1a68f5bc909146e9689c5428
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/mainwindow.h

@@ -78,7 +78,7 @@ public:
     MainWindow();
     ~MainWindow();
 
-    bool init(QString *errorMessage);
+    void init();
     void extensionsInitialized();
     void aboutToShutdown();