Deprecate E0706

compiler/rustc_error_codes/src/error_codes/E0706.md

@@ -1,8 +1,10 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 `async fn`s are not yet supported in traits in Rust.
 
 Erroneous code example:
 
-```compile_fail,edition2018
+```ignore,edition2018
 trait T {
     // Neither case is currently supported.
     async fn foo() {}
@@ -13,7 +15,7 @@ trait T {
 `async fn`s return an `impl Future`, making the following two examples
 equivalent:
 
-```edition2018,ignore (example-of-desugaring-equivalence)
+```ignore,edition2018 (example-of-desugaring-equivalence)
 async fn foo() -> User {
     unimplemented!()
 }