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
Maybe I don't have my head around this PR yet, but why would it be broken? We shouldn't be making backwards-incompatible changes to v1 APIs.