theme/panel: Reduce spacing between Activities and appmenu

As pointed out in #3769 (closed), this removes the spacing between the appMenu and the Activities in the panelLeft element. Since we do not need any spacing, the panelLeft element is removed altogether since it is unlikely we will need to add spacing in the future. Also removes the comment since it is mostly redundant for panelCenter which remains

Fixes #3769 (closed)

Screenshots :-

Before

After