update README for Miri not being in toolstate any more

Author: RalfJung

README.md

@@ -8,7 +8,6 @@ The latest status can be read from https://rust-lang-nursery.github.io/rust-tool
 
 Currently these tools are tracked:
 
-* [miri](https://github.com/rust-lang/miri)
 * All the external books
 
 These tools can be in one of the following states: