pretty sure we don't need this any more.

It doesn't look like the supplied context is ever modified.

This is only called in three places. Two of them don't pass `auth_events`, and
the third doesn't use the dict after passing it in, so this should be non-functional.

`_check_event_auth` is only called in three places. `on_send_membership_event`
doesn't pass an `auth_events`, and `prep` and `_auth_and_persist_event` do not
use the map after passing it in.

Return the updated auth event dict, rather than modifying the parameter.

This is only called from `_check_event_auth`.

Rename `auth_events` parameter to better reflect what it contains.

rename `auth_events` parameter to better describe what it contains