Remove unnecessary weak_oops_do call in adjust weak roots phase

In `G1FullGCAdjustTask`, the following code is for updating pointers to the new locations of objects. This is after ref-processing phase, so discovered lists are empty; all elements are either dropped or pushed onto the pending list. Therefore, the `weak_oops_do` call is a no-op.

```
  // Adjust the weak roots.
  if (!Atomic::cmpxchg(&_references_done, false, true)) {
    G1CollectedHeap::heap()->ref_processor_stw()->weak_oops_do(&_adjust);
  }

  AlwaysTrueClosure always_alive;
  _weak_proc_task.work(worker_id, &always_alive, &_adjust);
```

The Serial and Parallel collector also have the same issue, in `genCollectedHeap.cpp` and `psParallelCompact.cpp` respectively.