js: Remove unnecessary versions of clutter_actor_get_children

clutter_actor_get_children requires making a temporary GSList from
a linked list structure, and then creating a JS Array from that GSList.
For simple cases like the number of children, use clutter_actor_get_n_children.

https://bugzilla.gnome.org/show_bug.cgi?id=677426

js/ui/overview.js

@@ -14,7 +14,6 @@ const AppDisplay = imports.ui.appDisplay;
 const ContactDisplay = imports.ui.contactDisplay;
 const Dash = imports.ui.dash;
 const DND = imports.ui.dnd;
-const Lightbox = imports.ui.lightbox;
 const Main = imports.ui.main;
 const MessageTray = imports.ui.messageTray;
 const Panel = imports.ui.panel;