Remove vicious hack for runtime-calculated state-ness in derived type descriptors.

See `rust_crate_cache::get_type_desc`, in particular the line

```
td->is_stateful |= descs[i]->is_stateful;
```

This is clearly not the correct way to decide whether the derived type is itself stateful.

It isn't clear whether this ought to be fixed as is or should remain open pending unsettled deliberations about the mutability system.
