Mark E0399 test as obsolete

src/librustc_error_codes/error_codes/E0399.md

@@ -1,9 +1,11 @@
+Note: This error code is no longer emitted by the compiler.
+
 You implemented a trait, overriding one or more of its associated types but did
 not reimplement its default methods.
 
 Example of erroneous code:
 
-```compile_fail,E0399
+```ignore
 #![feature(associated_type_defaults)]
 
 pub trait Foo {
@@ -20,7 +22,7 @@ impl Foo for i32 {
 
 To fix this, add an implementation for each default method from the trait:
 
-```
+```ignore
 #![feature(associated_type_defaults)]
 
 pub trait Foo {