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
It's actually not, it exists in both /search/ and /fireplace/

By broken I mean "useless". We aren't using rocketfuel collections anymore, so anything returned in those 3 arrays is not matching anything the users see on the website. Which is why I'm tweaking it to return empty arrays.

But it means the endpoint is completely redundant with the regular search endpoint now. Hence the reason for removing it from the docs.