Simplify end_card calculation in G1BlockOffsetTablePart::verify

In `G1BlockOffsetTablePart::verify()`, we verify cards up to `end_card`:

```
size_t next_offset_index = _bot->index_for_raw(_next_offset_threshold);
size_t end_card = MIN2(_bot->index_for(_hr->top() - 1), next_offset_index - 1);
```

For regions with valid BOT, `_top <= _next_offset_threshold` holds, meaning each object has its corresponding block.

Therefore, we can use `_top` alone to determine `end_card`.