G1: Remove unnecessary check in G1Policy::calculate_desired_eden_length_by_mmu

In the sole caller of `G1Policy::calculate_desired_eden_length_by_mmu`:

```
  if (use_adaptive_young_list_length()) {
    desired_eden_length_by_mmu = calculate_desired_eden_length_by_mmu();
```

so, one can assert `use_adaptive_young_list_length()` as a precondition, instead of checking for it again.