How we can remove "Check All"

Hi,
How we can remove "Check All" in dropdown. i tried with showCheckAll: false.

this.dropdownSettings = {
      singleSelection: false,
      showCheckAll: false,
      showUncheckAll: false,
      idField: 'item_id',
      textField: 'item_text',
      itemsShowLimit: 1,
      allowSearchFilter: false
    };

I tried with  `enableCheckAll: false` also.