`PCReferenceProcessor` effectively contains

```
return PSParallelCompact::mark_bitmap()->is_unmarked(referent)
        && ReferenceProcessor::discover_reference(obj, type);
```

on deciding whether a non-strong reference should be discovered. However, `ReferenceProcessor::discover_reference` already checks the liveness of the referent. Therefore, `is_unmarked` is not needed, the whole class `PCReferenceProcessor` can be dropped.