Remove code no longer relevant because of the feed (bug 1058292)

https://bugzilla.mozilla.org/show_bug.cgi?id=1058292

- 'categories' API endpoint is useless
- /search/featured/ returns rocketfuel collections, which are going away
- header.js has been replaced by navbar.js
- 'collection' API endpoint and associated view/template are obsolete now that we have the feed.