dataraces: don't print an uninformative "Data race" bug msg

dataraces: don't print an uninformative "Data race" bug msg

Now that checkDataRaces() handles all bug reporting and assertions on
its own, we don't need an additional bug report message printed by the
caller. In some cases, this means the caller doesn't have any action to
take. In one case (a race immediately-realized from a user thread), we
still need to "bail out" to the model-checker.