QComboBoxListView: setAutoFillBackground to false

Fixes combobox dropdown not having outlines like it used to in Plasma 5.

The reason this happens is that when using setAutoFillBackground, the ComboBoxListView background color covers the whole frame.

| Before | After |
| ------ | ------ |
|    ![image](/uploads/242802dd5247e26f4557aeaf94113f44/image.png)    |    ![image](/uploads/855bc330afaf112c25dff5e0b96f6103/image.png)    |


(cherry picked from commit 973bc7f5)