appDisplay: Don't emit 'apps-changed' when deleting folder

To delete a folder, FolderView needs to reset all keys under that particular
folder's GSettings path. That generates 5 'changed' signals, all of which
end up calling AppDisplay._redisplay(), which is costly.

Don't emit 'apps-changed' when deleting a folder.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1447