Designer: Remove extra contents margins in the settings dialog

Task-number: QTCREATORBUG-29108
Change-Id: If3c6b0bcf9e1f05705c7e85fd814740235dbc7a6
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/designer/settingspage.cpp

@@ -25,6 +25,7 @@ public:
     {
         auto vbox = new QVBoxLayout(this);
         vbox->addWidget(m_designerPage->createPage(nullptr));
+        vbox->setContentsMargins({});
     }
 
     void apply() { m_designerPage->apply(); }