[Bug Report] while remove mutile remote selected options of el-select component by backspace, remove-tag event is not trigger

### Element UI version
2.13.0

### OS/Browsers version
window10/ chrome77.0.3865.120

### Vue version
2.5.20

### Reproduction Link
https://codepen.io/tanglinhai/pen/MWYvmyv?editors=1111

### Steps to reproduce
1、Enter the character a in the input box, and then select several drop-down values in the drop-down options
2、Then position the cursor at the end of the input box, and use backspace to delete a selected tag.
3、Finally, check the console. The usertagdel event is not executed in the console, so several console.logs in the usertagdel are not executed either.

### What is Expected?
1、Just like clicking the delete button above the tag, deleting the tag through the backspace key should also call back the remove tag event.

### What is actually happening?
deleting the tag through the backspace key the remove tag event is not call back .