popupMenu: Hide ornament by default

popupMenu: Hide ornament by default

We have always defaults to an empty ornament, so that menu items
are always aligned, even when radio items are used.

However radio items are fairly rare, so most of the time we end
up with an extra margin with no purpose. The design team now
prefers radio items to only align with each other, so that regular
items get the expected margin.

Change the defaults accordingly.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2843>