readme: remove paragraph about executing clippy via "cargo run .."
readme: remove paragraph about executing clippy via "cargo run .."

This most likely no longer works since we are pinning clippy on a specific nightly now.
"cargo run" would try to compile clippy with whatever version the project we want to check demands.

Also building clippy yourself to run it on a project is not really needed anymore since clippy is shipped with official rust releases.

Fixes #6489

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