Remove redundancy in ReferenceProcessor constructor args

```
ReferenceProcessor::ReferenceProcessor(BoolObjectClosure* is_subject_to_discovery,
                                       uint mt_processing_degree,
                                       bool mt_discovery,
                                       uint mt_discovery_degree,
                                       bool concurrent_discovery,
                                       BoolObjectClosure* is_alive_non_header)
```

`mt_discovery` is effectively `mt_discovery_degree > 1`, so one can drop `bool mt_discovery` in the arg list.