Indicate E0665 is no longer emitted

Author: jhpratt

compiler/rustc_error_codes/src/error_codes/E0665.md

@@ -1,8 +1,10 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 The `Default` trait was derived on an enum.
 
 Erroneous code example:
 
-```compile_fail,E0665
+```compile_fail
 #[derive(Default)]
 enum Food {
     Sweet,