easyui datagrid将表头的checkbox不显示（隐藏）

<script type="text/javascript">
    $(function(){
        $("#dg").datagrid({
            url:"../json/content.json",
            //url:"../student.php",
            striped:true,
            width:400,
            loadMsg:"正在加载中。。。。",
            singleSelect:true,
            title:"用户列表",
            columns:[[
                {
                    field:"name",
                    title:"账号"
                } ,
                {
                    field:"email",
                    title:"邮箱"
                } ,
                {
                    field:"date",
                    title:"创建时间"
                }
            ]],
            frozenColumns:[[
                {field:'ck',title:"da",checkbox:true}
            ]],
            toolbar: [{
                iconCls: 'icon-edit',
                handler: function(){alert('编辑按钮')}
            },'-',{
                iconCls: 'icon-help',
                handler: function(){alert('帮助按钮')}
            }],
            onLoadSuccess: function () {   //隐藏表头的checkbox
                $("#dg").parent().find("div .datagrid-header-check").children("input[type=\"checkbox\"]").eq(0).attr("style", "display:none;");
            }
        });
    });
 
</script>