rustc_mir: remove outdated `const fn` stability diagnostic.
rustc_mir: remove outdated `const fn` stability diagnostic.

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