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

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

src/client/conn.rs

@@ -182,7 +182,6 @@ where
     /// # Example
     ///
     /// ```
-    /// # #![feature(async_await)]
     /// # use http::header::HOST;
     /// # use hyper::client::conn::SendRequest;
     /// # use hyper::Body;

src/client/connect/http.rs

@@ -46,7 +46,6 @@ pub struct HttpConnector<R = GaiResolver> {
 /// # Example
 ///
 /// ```
-/// # #![feature(async_await)]
 /// # async fn doc() -> hyper::Result<()> {
 /// use hyper::Uri;
 /// use hyper::client::{Client, connect::HttpInfo};

src/client/mod.rs

@@ -27,7 +27,6 @@
 //! [full client example](https://github.com/hyperium/hyper/blob/master/examples/client.rs).
 //!
 //! ```
-//! # #![feature(async_await)]
 //! use hyper::{Client, Uri};
 //!
 //! # #[cfg(feature = "runtime")]