[WidgetExplorer] Don't always show vertical sidebar
In the past, this was done to avoid horizontally resizing delegates which could generate a situation in which the view switched rapidly between being scrollable and non-scrollable.
However since then, it appears that this quirk has been resolved by other layout and animation changes, as I can no longer reproduce it.
Accordingly, we can remove the hack and make the scrollbar disappear when the view isn't scrollable now.
BUG: 426077 FIXED-IN: 5.21.3