Final deprecation for constant BADGE_BACKPACKURL (it has been removed)
and badges/backpack.js file (it has been removed too).
Besides, some methods have started the deprecation process too:
 - badges_check_backpack_accessibility
 - badges_setup_backpack_js()
 - badges_local_backpack_js()

Apart from that, string 'addbackpack' has been deprecated here
because it's not used anywhere.