G1: Remove redundant checks in G1CardSet::free_mem_object

In `G1CardSet::free_mem_object`:

```
  assert(...);
  ...
  if (type == G1CardSet::CardSetArrayOfCards ||
      type == G1CardSet::CardSetBitMap ||
      type == G1CardSet::CardSetHowl)
  ...
```

The same checks are asserted right above.