Drop format URI parameter

[christianlupus]

The ?format= is pretty bad IMO, since it does not adhere to HTTP standards while the Accept header is exactly made for this purpose. I'm not even sure we should explain here that such a query parameter ever existed.

Originally posted by @provokateurin in #12549 (comment)

Should we drop this section about the ?format= then completely, mark it as deprecated or just let it be as it is right now?