Don't use custom PopoverContainer in status area applet

This type seems unnecessary now that
https://docs.gtk.org/gtk4/method.MenuButton.set_child.html exists. And
in any case, isn't needed here.

applets/cosmic-applet-status-area/src/main.rs

@@ -2,7 +2,6 @@ use gtk4::{glib, prelude::*};
 
 mod dbus_service;
 mod deref_cell;
-mod popover_container;
 mod status_area;
 mod status_menu;
 mod status_notifier_watcher;