[Places Panel] Disallow editing devices

I introduced this bug when I changed the layout of the menu.
While it would be nice if one could edit devices, it is currently not possible
and would require significant re-engineering effort in Solid.

While the "Edit" menu works just fine, changes are not persisted.

Reviewed-By: emmanuelp

Differential Revision: https://phabricator.kde.org/D6294