Remove future re-exports (#133)

These types were moved around in
https://github.com/tokio-rs/axum/pull/130 but re-export from their old
location for backwards compatibility.

This removes the re-exports.

src/handler/mod.rs

@@ -4,7 +4,7 @@ use crate::{
     body::{box_body, BoxBody},
     extract::FromRequest,
     response::IntoResponse,
-    routing::{EmptyRouter, MethodFilter, RouteFuture},
+    routing::{future::RouteFuture, EmptyRouter, MethodFilter},
     service::HandleError,
 };
 use async_trait::async_trait;