Remove forwarding headers

[dbolduc]

Some services were added with unversioned clients. We generated versioned clients. To maintain backwards compatibility, we pointed the unversioned headers to versioned headers, and unversioned clients to versioned clients. See #10170 for more background.

The unversioned types have already been deprecated.

We should remove the headers and types before our next release. e.g. https://github.com/googleapis/google-cloud-cpp/blob/36d368d6d3128942da6d30cf204369ee06f51e6f/generator/generator_config.textproto#L23