Remove old_io from trpl/concurrency.md

[callahad]

Also: the std_misc feature flag is removed; it's not needed in Beta.

Hat tip to @tshepang in #23871

Fixes#24023

This PR replaces #24024

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @pcwalton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see CONTRIBUTING.md for more information.

[callahad]

Doh, forgot about @rust-highfive's auto-assignment.

Per Steve's comment in #24024:

r? @steveklabnik

[steveklabnik]

@bors: r+ rollup

[bors]

📌 Commit 3b73a6e has been approved by steveklabnik

[steveklabnik]

Thank you!

In the future, you don't need a new PR to do this: http://blog.steveklabnik.com/posts/2012-11-08-how-to-squash-commits-in-a-github-pull-request

[callahad]

Oh, really? GitHub handles a force push like that intelligently? Awesome!

[steveklabnik]

Yup! :)

[Manishearth]

Needs Manishearth@b62c110 to pass

[callahad]

@Manishearth I'm still super unfamiliar with the Rust contrib process. Do I need to take any action (adding / squashing your commit into this PR), or is your fixup going to get handled elsewhere?

[Manishearth]

Oh, sorry.

So your PR is included in the current rollup (big ball of PRs to push through the CI), including this fix. If all goes well you don't have to worry.

However, if the rollup fails or is abandoned, the next time this gets built it will fail.

I suggest you cherrypick that commit into your PR and leave it like that :)

[Manishearth]

(It merged now, so you don't need to do anything)