code: function (zlevel) {
          var layers = this._layers;
          var zlevelList = this._zlevelList;
          var layer = layers[zlevel];
          if (!layer) {
              return;
          }
          layer.dom.parentNode.removeChild(layer.dom);
          delete layers[zlevel];
  
          zlevelList.splice(util.indexOf(zlevelList, zlevel), 1);
      }
docs: 删除指定层
  @param {number} zlevel 层所在的zlevel