We only want to emit the deprecation warning in case a user uses
the old features and _not_ the new ones. This assumes that a user
upgrades properly, i.e. adds the new `tokio` or `async-std` feature
AND removes the old `tcp-tokio` feature from the list.

We need to use this rather complex cfg expression because otherwise,
our CI won't pass. We run clippy with `--all-features`, thus just
detecting for presence of the `tcp-async-io` feature for example
will always flag our examples as using deprecated code.

Otherwise CI will not pass once #2963
actually checks it properly.