RoomAssistance: Don't send "withdrawn" notifications for past events

They are just noise and break e.g. event deletion during category cleanup (*see issue 1052 on sentry*), since there's no session timezone information available at that point to format the timestamp in the email.

Inspect the RoomAssistance code that sends "withdrawn" notifications and trace the event-deletion path involved in category cleanup. Reproduce the past-event case described in the issue, then verify that deleting or cleaning up such events no longer sends a withdrawn notification or requires session timezone information.

Written by the indexing model from the issue text.