zstd library dependency still needed ?

There is a dependency on zstd https://github.com/google/xls/blob/main/dependency_support/load_external.bzl#L62-L73

I was considering adding the additional targets in our [BUILD file for zstd](https://github.com/google/xls/blob/main/dependency_support/com_github_facebook_zstd/bundled.BUILD.bazel) and upstreaming it to the BZR version https://registry.bazel.build/modules/zstd ...

However, right now it looks like it is not referenced anywhere in our BUILD files, so maybe we don't need this anymore ? Is all the zstd testing done using riegeli ?

I might be wrong though, there is a pending PR https://github.com/google/xls/pull/3636 so maybe that needs that dependency.
Tagging @magancarz (author of that PR) and @proppy (reviewer?)