Remove scrollbar groove completely

Seems this was not cherrypicked for Plasma 6.0.

Alternative for !401 (closed)

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

Note: Plasma Theme needs this also removed!

Why:

Because this looks awfully confusing:

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

(cherry picked from commit a6327c06)

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