chore(lib): remove async_await feature gate (#1909)

`async_await` is stabilized in rust-lang/rust#63209.

src/service/make_service.rs

@@ -133,7 +133,6 @@ where
 /// # Example
 ///
 /// ```rust,no_run
-/// # #![feature(async_await)]
 /// # #[cfg(feature = "runtime")]
 /// # #[tokio::main]
 /// # async fn main() {

src/service/service.rs

@@ -70,7 +70,6 @@ mod sealed {
 /// # Example
 ///
 /// ```rust
-/// # #![feature(async_await)]
 /// use hyper::{Body, Request, Response, Version};
 /// use hyper::service::service_fn;
 ///