Drop level="top" attribute from panels

The cui-widget-panel class does not effect the panels we add it to. And there doesn't seem to be any reason to hard set the level attribute, rather than using the default.

For example, the site identity panel doesn't use either.

Update: cui-widget-panel actually does apply the font: menu rule, which has an effect on MacOS. The identity panel applies the same rule, but without using the cui-widget-panel class. I checked with firefox developers and it seems we do want this. Mozilla might change this to just apply to all panels in the future though, rather than using this class.