[Bug Report] The table column which type is "selection" can not cancel fixed when all the column is not fixed

### Element UI version
2.13.2

### OS/Browsers version
MacOs big sur(11.6 (20G165)) / Chrome 版本 96.0.4664.110（正式版本） (arm64)

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/myGitHubForJava/pen/mdBzpww

### Steps to reproduce
1. The table is not fixed firstly, ths first column is fixed after the button is clicked, and the column which type is "selection" is also be fixed
2. Then click the button to unfix the column, but the column which type is "selection" can not be unfixed.

### What is Expected?
When all the column is not fixed, the column which type is "selection" also should be unfixed

### What is actually happening?
the column which type is "selection" always be fixed