relax bounds on free fn

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tower/src/util/future_service.rs

@@ -48,7 +48,6 @@ use tower_service::Service;
 pub fn future_service<F, S, R, E>(future: F) -> FutureService<F, S>
 where
     F: Future<Output = Result<S, E>> + Unpin,
-    S: Service<R, Error = E>,
 {
     FutureService::new(future)
 }