ProjectExplorer: Consolidate creation of KitAspect's "Manage..."

Change-Id: I001a21f41bcd5ff0614bdf98893b56137c06546f
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/mesonprojectmanager/toolkitaspectwidget.h

@@ -40,7 +40,6 @@ private:
     {
         addMutableAction(m_toolsComboBox);
         parent.addItem(m_toolsComboBox);
-        parent.addItem(m_manageButton);
     }
 
     void refresh() override
@@ -58,7 +57,6 @@ private:
     }
 
     QComboBox *m_toolsComboBox;
-    QWidget *m_manageButton;
     ToolType m_type;
 };