This commit makes the `Config.secret_key` conditionally compile on the
`secrets` feature. The net effect is simplified internal code, fewer
corner-cases, and easier to write tests.
This commit removes the `Provider::profile()` implementation of
`Config`. This means that the `Config` provider no longer sets a
profile, a likely confusing behavior. The `Config::figment()` continues
to function as before.
Sergio Benitez