Improve error messages with #[diagnostic::do_not_recommend] (#3588)

axum-macros/tests/from_request/fail/generic_without_via.stderr

@@ -13,9 +13,6 @@ error[E0277]: the trait bound `fn(Extractor<()>) -> impl Future<Output = ()> {fo
    |                                        required by a bound introduced by this call
    |
    = note: Consider using `#[axum::debug_handler]` to improve the error message
-   = help: the following other types implement trait `Handler<T, S>`:
-             `Layered<L, H, T, S>` implements `Handler<T, S>`
-             `MethodRouter<S>` implements `Handler<(), S>`
 note: required by a bound in `axum::routing::get`
   --> $WORKSPACE/axum/src/routing/method_routing.rs
    |