How do I remove/ dont display the Custom option from Rad Grid View Column -> Available Filters list?

        public RadForm1()
        {
            InitializeComponent();

            this.radGridView1.EnableFiltering = true;
            this.radGridView1.ShowHeaderCellButtons = true; 
            this.radGridView1.FilterPopupInitialized += RadGridView1_FilterPopupInitialized;
        }

        private void RadGridView1_FilterPopupInitialized(object sender, FilterPopupInitializedEventArgs e)
        {
            RadListFilterPopup popup = e.FilterPopup as RadListFilterPopup;
            if (popup!=null)
            {
                foreach (RadItem item in popup.Items)
                {
                    if (item.Text == RadGridLocalizationProvider.CurrentProvider.GetLocalizedString(RadGridStringId.FilterMenuAvailableFilters))
                    {
                        RadMenuItem availableFilters = item as RadMenuItem;
                        foreach (RadItem subItem in availableFilters.Items)
                        {
                            if (subItem.Text == RadGridLocalizationProvider.CurrentProvider.GetLocalizedString(RadGridStringId.FilterOperatorCustom))
                            {
                                subItem.Visibility = ElementVisibility.Collapsed;
                            }
                        } 
                    } 
                }
            }
        }