Update compile-fail test

Author: oli-obk

src/test/compile-fail/const-fn-error.rs

@@ -14,8 +14,6 @@ const X : usize = 2;
 
 const fn f(x: usize) -> usize {
     let mut sum = 0;
-    //~^ let bindings in constant functions are unstable
-    //~| statements in constant functions are unstable
     for i in 0..x {
         //~^ ERROR E0015
         //~| ERROR E0019