Stabilize the compile_error_macro feature

Stabilizes:

* `compile_error!` as a macro defined by rustc

Closes #40872

src/test/compile-fail/compile_error_macro.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(compile_error)]
-
 fn main() {
     compile_error!("a very descriptive error message"); //~ ERROR: a very descriptive error message
 }