Reinstate the error-code error over the feature gate error

compiler/rustc_error_codes/src/error_codes/E0492.md

@@ -3,7 +3,6 @@ A borrow of a constant containing interior mutability was attempted.
 Erroneous code example:
 
 ```compile_fail,E0492
-#![feature(const_refs_to_cell)]
 use std::sync::atomic::AtomicUsize;
 
 const A: AtomicUsize = AtomicUsize::new(0);
@@ -31,7 +30,6 @@ static B: &'static AtomicUsize = &A; // ok!
 You can also have this error while using a cell type:
 
 ```compile_fail,E0492
-#![feature(const_refs_to_cell)]
 use std::cell::Cell;
 
 const A: Cell<usize> = Cell::new(1);