Weakable is not thread-safe, so taking a strong reference from a
WeakPtr<Thread> may result in a use-after-free. We don't use this
functionality anywhere anyway, so remove it.
- `Threading::Thread` is not polymorphic, there is no need for a virtual
destructor.
- `HTMLAnchorElement::has_download_preference` isn't overridden by
anything.
This warning was introduced in llvm/llvm-project#131188.