[Bug Report] when el-table-column's type is selection,the attribute width value set for the column is 30px, and a dot symbol will appear in the lower right corner of the check box on the page

### Element UI version
2.13.1

### OS/Browsers version
Windows 10 / Chrome v83.0.4103.116 x64

### Vue version
2.6.11

### Reproduction Link
https://github.com/shilohooo/element-bug-recurrent/blob/master/views/table-test.html

### Steps to reproduce
1. add vue and element cdn to a base html file
2. create a vue instance and bind it to a div with id attribute as app
3. create a table data list
4. Create a table element and bind the list to the table's data attribute
5. Create a el-table-column element and set the type attribute to selection
6. Set the width of the el-table-column element to 30px
7. Open the web page and you will see the effect of bug reproduction

### What is Expected?
Only checkboxes are displayed in column elements

### What is actually happening?
In addition to the check box, there is a dot symbol displayed in the lower right corner of the column element:(