Cleanup README for clippy-preview on stable
Cleanup README for clippy-preview on stable

With the 1.29 release, the `clippy-preview` component will be available
on stable which means we don't need nightly/beta anymore.

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust