Update CONTRIBUTING now that the book is not here

Author: ehuss

CONTRIBUTING.md

@@ -6,11 +6,3 @@
 > on [Zulip] if you are interested in participating.
 
 [Zulip]: https://rust-lang.zulipchat.com/#narrow/stream/399173-t-spec
-
-## Building
-
-See the [Building docs](README.md#building) for information on generating HTML from the specification source files.
-
-## Authoring guide
-
-See the [Authoring Guide](docs/authoring.md) for information about working with the specification source files.