Table scroll mofify scrollbar width

### Element UI version
2.13.1

### OS/Browsers version
window10/ chome 75.0.3770.142

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/mrsbetter/pen/JjYRGzP

### Steps to reproduce
直接打开即可看到

### What is Expected?
gutter宽度，及scroll宽度变小时，不要有默认的16px给滚动条

### What is actually happening?
table的scrollbar宽度变小，表格中的scrollbar宽度撑开的宽度还是16px