[Bug Report] Memory leak at el-tooltip cleanup

### Element UI version
0.1.0

### OS/Browsers version
All

### Vue version
3.0.0-beta.10

### Reproduction Link
https://github.com/ElemeFE/element/blob/dev/packages/tooltip/src/main.js

### Steps to reproduce
Traced a memory leak to this to the code:
https://github.com/ElemeFE/element/blob/dev/packages/tooltip/src/main.js

in the mounted there is a 'focus' event listener registered to lambda. In the destroyed we try to to clean it by referencing 'this.handleFocus' but that is not the function that is bound to the event.

### What is Expected?
The component event listener should be cleaned up properly.

### What is actually happening?
Event is not cleaned up and we leak memory.