[Bug Report] el-select dropdown 的 z-index 增加2每次点击的时候

### Element UI version
2.13.2

### OS/Browsers version
Mac os

### Vue version
2.6.7

### Reproduction Link
https://codepen.io/pgclrqvj/pen/eYdmZEp

### Steps to reproduce
1. 点击el-select。
2. 用chrome的调试工具，看一看有class="el-select-dropdown el-popper"的元素。
3. 它的z-index增加2每次点击的时候。

1. click el-select.
2. with chrome developer's tool, inspect element with "el-select-dropdown el-popper" class.
3. see its z-index increaes 2 whenever gets clicked.

### What is Expected?
dropdown menu还原的时候z-index也要还原。

when dropdown menu get closed, increased z-index should be decrease so that it would be the same as before clicked.

### What is actually happening?
有class="el-select-dropdown el-popper"的元素的z-index继续增加。

z-index of the element with "el-select-dropdown el-popper" class increases 2, never decreases.