QmlDesigner: Don't update sub component manager when adding a module

It seems there is no need to update it and it is an expensive
operation.

Task-number: QDS-4008
Change-Id: I5dd8427f73a6ce755b017f6d284b72acb31ed7d5
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/plugins/qmldesigner/components/itemlibrary/itemlibrarywidget.cpp

@@ -292,7 +292,7 @@ void ItemLibraryWidget::handleAddImport(int index)
     }
 
     m_model->changeImports({import}, {});
-    QmlDesignerPlugin::instance()->currentDesignDocument()->updateSubcomponentManager();
+
     m_stackedWidget->setCurrentIndex(0); // switch to the Components view after import is added
     updateSearch();
 }