Remove old TOC item

Author: camelid

CONTRIBUTING.md

@@ -15,7 +15,6 @@ All contributors are expected to follow the [Rust Code of Conduct].
 - [Contributing to Clippy](#contributing-to-clippy)
   - [Getting started](#getting-started)
     - [Finding something to fix/improve](#finding-something-to-fiximprove)
-    - [How to build Clippy](#how-to-build-clippy)
   - [Writing code](#writing-code)
   - [Getting code-completion for rustc internals to work](#getting-code-completion-for-rustc-internals-to-work)
   - [How Clippy works](#how-clippy-works)