去除gridPanel表头的全选框

去掉gridPanel表头的全选框
给gridpanel 加上listener即可

,listeners:{
render:function(){
var hd_checker = this.getEl().select('div.x-grid3-hd-checker');
if (hd_checker.hasClass('x-grid3-hd-checker')) { 
     hd_checker.removeClass('x-grid3-hd-checker');
  }
}
}