Kill broken and unused `FederationServer.on_query_auth_request`

[clokep]

We talked a bit about FederationBase calling a FederationClient method on Matrix, wanted to capture the results from rich:

I'm pretty sure I decided that FederationServer.on_query_auth_request had been broken forever and needed to die, which means that _check_sigs_and_hash_and_fetch can move to FederationClient
some more context: _on_query_auth_request implements an undocumented, unused REST endpoint

Originally posted by @clokep in #6996 (comment)

[clokep]

While researching this it seems it is actually specced: https://matrix.org/docs/spec/server_server/r0.1.3#post-matrix-federation-v1-query-auth-roomid-eventid

We think it is unused, but will need to check that. We'll also need to write an MSC to remove it from the protocol.

[clokep]

Note that the implementation of these functions right now is confusing. FederationBase calls get_pdu, which is actually part of FederationClient.

In order to move this code to FederationClient the query_auth endpoint needs to be cleaned up.

[clokep]

It looks like the client portion of this was removed in #6214 as part of 1.5.0rc1.

[clokep]

See matrix-org/matrix-spec-proposals#2451.

[clokep]

Fixed by #7026.