Parallel: Simplify CLD roots claim in Full GC cycle

`SequentialSubTasksDone` is used to claim tasks:

```
    for (uint task = 0; _subtasks.try_claim_task(task); /*empty*/ ) {
      mark_from_roots_work(static_cast<ParallelRootType::Value>(task), worker_id);
    }
```

However, there's actually only one task here, `ParallelRootType::class_loader_data`, and CLD has its own internal claiming mechanism.