G1: Remove redundant checks in check_obj_during_refinement

In `check_obj_during_refinement`:

```
  assert(g1h->is_in_reserved(obj), ... // <- this can be strenghen to `is_in`
  
  HeapRegion* from = g1h->heap_region_containing(p);
  assert(from->is_in_reserved(p) || (from->is_humongous() ... // <- this can be replaced by `is_in`
```