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
I don't think it's correct to remove this, since it still exists in the `API_CURRENT_VERSION`. It might be better to `.. note::` that it's removed in v2 and have the URL reflect that.