Remove 1.23.0 release notes entry on const_fn

It doesn’t change anything for stable users in practice. See discussion in https://github.com/rust-lang/rust/pull/46148

RELEASES.md

@@ -5,10 +5,6 @@ Language
 --------
 - [`non_snake_case` lint now allows extern no-mangle functions][44966]
 - [Now accepts underscores in unicode escapes][43716]
-- [`#![feature(const_fn)]` is now no longer required for
-  calling const functions.][43017] It's still required for creating
-  constant functions, and const functions in the standard libraries
-  now have individual feature gates.
 - [`T op= &T` now works for numeric types.][44287] eg. `let mut x = 2; x += &8;`
 - [types that impl `Drop` are now allowed in `const` and `static` types][44456]