Primiitive的删除，同样用于PrimitiveCollection.

let react = scene.primitives.add(new Cesium.RectanglePrimitive({
    //绘制矩形
    rectangle : Cesium.Rectangle.fromDegrees(-100.0, 20.0, -90.0, 30.0),
    material : Cesium.Material.fromType('Dot')  //设置材质
}));

//根据变量销毁
react.destroy();

//销毁所有
scene.primitives.removeall(),//谨慎使用，可能删除不必要的primitive


//删除Collection 
this.chinaBoundary = new Cesium.PrimitiveCollection({
      destroyPrimitives: false
});

viewer.scene.primitives.remove(this.chinaBoundary);
this.chinaBoundary.removeAll();

entity的删除，同样用于entityCollection.

// 根据entity（实体）对象删除
viewer.entities.remove(entity_obj)
// 根据ID删除
viewer.entities.removeById(entity_id)
// 删除全部
viewer.entities.removeAll()


// 根据entity的ID 获取 entity实体对象
var entity_obj = viewer.entities.getById(entity_id)