Drop CPS-specific threading guidance (#1445)

This guidance is long outdated.

Author: AArnott

docfx/docs/cookbook_vs.md

@@ -1,11 +1,6 @@
 Cookbook for Visual Studio
 ==========================
 
-Important for CPS extension authors and clients: please
-replace all references to `ThreadHelper.JoinableTaskFactory` with
-`this.ThreadHandling.AsyncPump`, where `this.ThreadHandling` is an `[Import]
-IThreadHandling`.
-
 ## Initial setup
 
 - Add a reference to [Microsoft.VisualStudio.Threading][NuPkg].