Purge Traceable and Untraceable from Servo

Now that we use `JSTraceable` (defined in `script`), we can create arbitrary implementations on non-`script` types (eg `Url` or `RequestHeaderCollection`) where in the past we had to rely on `Traceable` and `Untraceable` to achieve cross-crate impls of `Encodable`.

This removes the two completely. They can be reintroduced if required, though the `untraceable!` macro should suffice.

Fixes #3469