Replace collection data in /api/v1/search/featured/ by empty arrays (bug...

Also simplify fireplace version now that yogafire - which doesn't care about collections - is the only client left using it.

https://bugzilla.mozilla.org/show_bug.cgi?id=1058292
Yeah, I wasn't sure, but my reasoning was that we don't want people to use it since it'll be broken anyway as we are moving rocketfuel stuff (and, in this patch, returning empty arrays instead)