Remove scrollbar groove completely

Alternative for !401 

Since we already have the separator for scrollbar, we don't need the groove (the area that appears when hovering mouse over scrollbar field).

This simplifies the look and removes some unnecessary lines from the scrollbar field.

| Before | After |
| ------ | ------ |
|  ![image](/uploads/55a90ce930e1646f78642475634d228a/image.png)      |    ![image](/uploads/18e5f2d292e494c2d226393deb8e33d7/image.png)    |

Note: Plasma Theme needs this also removed!


Why:

Because this looks awfully confusing:

![image](/uploads/a695824e1bc25c23f20c2ba61348be62/image.png)

On top of that, when we add high contrast settings, it will look like this:

![image](/uploads/9ca7ef4916ea7bc7d198e0b509ab33db/image.png)


(cherry picked from commit a6327c06)

1ba56f8c Remove scrollbar groove completely
31ff325e Remove extra padding from scrollbar edges