Entries with a `last_status` older than 48h are not removed from the list

As @trinity-1686a points out we have a rather large part of our exit list containing entries that are weeks old. However, according to the spec this should not happen:

  "LastStatus" SP YYYY-MM-DD SP HH:MM:SS NL

    The last time, in UTC, when the relay was seen in a v2 network status
    document.  A relay is discarded from the list if it was not seen in a
    network status for at least 48 hours.