js: Remove modelines

We already include an .editorconfig that is supported by many
editors, including emacs, so no need to repeat an emacs-specific
modeline in every source file.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3431>

js/ui/popupMenu.js

@@ -1,5 +1,3 @@
-// -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
-
 import Atk from 'gi://Atk';
 import Clutter from 'gi://Clutter';
 import Gio from 'gi://Gio';