Remove ability to filter by platform from the search/autocomplete APIs

[diox]

Part of the removal of the platform field. #8174

We need to remove ability to filter by platform from the search/autocomplete APIs and decide what to do about backwards-compatibility.

[diox]

Note: frontend will stop exposing that filter in #13950

[diox]

Regarding backwards-compatibility: mozilla/addons-server#16524 will make it so there will only be one file per version, with the platform always set to all. So just ignoring the filter parameter should be enough - if there were filtering applied, it would have the exact same result.

[AlexandraMoga]

The ?platform query parameter is ignored in the /search/ and /autocomplete/ APIs, namely the same number of search results is returned no matter if the parameter is present or not.

This issue is verified fixed on stage.