lookingGlass: Remove Tweener from preimported modules

We want to encourage using Clutter's implicit animations directly,
so stop importing the discouraged animation framework by default.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/669

js/ui/lookingGlass.js

@@ -21,7 +21,6 @@ const CHEVRON = '>>> ';
 var commandHeader = 'const { Clutter, Gio, GLib, GObject, Meta, Shell, St } = imports.gi; ' +
                     'const Main = imports.ui.main; ' +
                     'const Mainloop = imports.mainloop; ' +
-                    'const Tweener = imports.ui.tweener; ' +
                     /* Utility functions...we should probably be able to use these
                      * in the shell core code too. */
                     'const stage = global.stage; ' +