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
Source-Repo: https://github.com/servo/servo
Source-Revision: b34df7c343579f200d2e67e21fc566842a4e4a91
