To my understanding, a bad user ID can sometimes make it into the
database, which lead to a panic prior to this change.

Remove it from request handler since there's already the context of the
request path, added through TraceLayer.

They are no longer needed in the 2021 edition.

Remove device_one_time_keys_count from is_empty() sync checks, fixing sync issue as reported by Nekron

Should make the code a little bit easier to read.

Co-authored-by: Timo Kösters <timo@koesters.xyz>