ui: Remove fallback app menu
The app menu is in the process of being retired[0], and the shell stopped displaying it while applications are in the process of dropping it. It therefore doesn't make sense to always show a fallback menu in server-side decorations, applications that still set the menu can rely on GTK+'s own fallback instead.
[0] https://gitlab.gnome.org/GNOME/Initiatives/wikis/App-Menu-Retirement