Remove thumbnail bar from CoverSwitch Effect

The usage has always been questionable as the thumbnails are
too small to properly recognize something.

Now BoxSwitch Effect has reach its end of life which would have
required to keep most of the code just for this thumbnail bar.

If required it would be better to integrate the normal QML based
TabBox to be rendered together with an effect.

BUG: 296070
FIXED-IN: 4.11.0