Remove reference to discontuned async-std package (#2841)

Fixes #2839.

Author: djmitche

src/concurrency/async/runtimes.md

@@ -11,8 +11,6 @@ not have a "built-in" runtime, but several options are available:
 - [Tokio](https://tokio.rs/): performant, with a well-developed ecosystem of
   functionality like [Hyper](https://hyper.rs/) for HTTP or
   [Tonic](https://github.com/hyperium/tonic) for gRPC.
-- [async-std](https://async.rs/): aims to be a "std for async", and includes a
-  basic runtime in `async::task`.
 - [smol](https://docs.rs/smol/latest/smol/): simple and lightweight
 
 Several larger applications have their own runtimes. For example,