Visible scrollbar when `dir="rtl"`

### Element UI version
2.12.0

### OS/Browsers version
Win 10 / Chrome latest

### Vue version
2.6.10

### Reproduction Link
https://jsfiddle.net/hksn7ra6/2/

### Steps to reproduce
1. use any of the select elements provided in the docs.
2. set `dir="rtl"` on `html` or `body` tag.

### What is Expected?
scrollbar should be hidden like when direction is left to right.

### What is actually happening?
scrollbar becomes visible by setting the direction to rtl.