Rollup merge of #72697 - ehuss:rm-rustc-ux-guidelines, r=nikomatsakis

Remove rustc-ux-guidelines

This is now in the rustc-dev-guide:
* rust-lang/rustc-dev-guide#716
* rust-lang/rustc-dev-guide#717

This is a public page, but it was not linked to anywhere, so I think it is safe to remove. Google searches don't show it being used anywhere.