G1: Remove redundant check in G1FlushHumongousCandidateRemSets

In `G1FlushHumongousCandidateRemSets`:

```
      if (!r->is_starts_humongous() ||
          !g1h->region_attr(r->hrm_index()).is_humongous() ||
          r->rem_set()->is_empty()) {
```

`!r->is_starts_humongous()` entails `!g1h->region_attr(r->hrm_index()).is_humongous()` already. The former can be dropped without affecting the filtering.