Remove async-task as a subject from the book

Link to the docs, the are extensive and useful.

docs/src/overview.md

@@ -4,8 +4,8 @@
 
 `async-std` provides an interface to all important primitives: filesystem operations, network operations and concurrency basics like timers. It also exposes `async-task` in a model similar to the `thread` module found in the Rust standard lib. The name reflects the approach of this library: it is a closely modeled to the Rust main standard library as possible, replacing all components by async counterparts. You can read more about `async-std` in [the overview chapter][overview-std].
 
-`async-task` is a library for implementing asynchronous tasks quickly. For the purpose of this documentation, you will mainly interact with it through the `async_std::task` module. Still, it has some nice properties to be aware of, which you can read up on in the [`async-task` overview chapter][overview-task].
+`async-task` is a library for implementing asynchronous tasks quickly. For the purpose of this documentation, you will mainly interact with it through the `async_std::task` module. Still, it has some nice properties to be aware of, which you can read up on in the [`async-task` crate docs][task-docs].
 
 [organization]: https://github.com/async-std/async-std
 [overview-std]: overview/async-std/
-[overview-task]: overview/async-task/
+[overview-task]: https://docs.rs/async-task

docs/src/overview/async-task.md

@@ -1 +0,0 @@
-# async-task