cleanup: simplify `GoogleDefaultCredentials()` implementation (#9600)
cleanup: simplify `GoogleDefaultCredentials()` implementation (#9600)

The helper functions had parameters that were effectively constant. And
we already have an `google::cloud::Options` parameter that we could
use for any configuration.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp