[bug report] El tree component, when clearing the selected state (call: this. $refs. Tree. Setcheckedkeys ([];), it is not clear

### Element UI version
2.13.0

### OS/Browsers version
win10/chrome 79.0.3945.88

### Vue version
3.0.0-alpha.4

### Reproduction Link
https://codepen.io/mshineray/pen/dyobpRZ

### Steps to reproduce
1、点击根节点全选
2、点击【清空】按钮，发现仍有两个复选框处于选中状态

### What is Expected?
点击清空后，全部处于未选中状态

### What is actually happening?
点击【清空】按钮，发现仍有两个复选框处于选中状态