Remove dead error code.

compiler/rustc_error_codes/src/error_codes/E0698.md

@@ -1,9 +1,11 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 When using generators (or async) all type variables must be bound so a
 generator can be constructed.
 
 Erroneous code example:
 
-```edition2018,compile_fail,E0698
+```edition2018,compile_fail,E0282
 async fn bar<T>() -> () {}
 
 async fn foo() {