JDK-8035406 renamed `_strong_code_roots_list` -> `_code_roots` in `HeapRegionRemSet`, but its relevant accessors still keep the "strong" prefix, e.g. `add_strong_code_root`.

Such inconsistency can cause some rather odd code, e.g. in `G1ScanCollectionSetRegionClosure`:

```
r->strong_code_roots_do(_pss->closures()->weak_codeblobs());
```

applying the weak closure on strong roots.

Drop the "strong" prefix to reconcile the names of the field and its accessors .