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

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

examples/client.rs

@@ -1,4 +1,3 @@
-#![feature(async_await)]
 #![deny(warnings)]
 extern crate hyper;
 extern crate pretty_env_logger;

examples/client_json.rs

@@ -1,4 +1,3 @@
-#![feature(async_await)]
 #![deny(warnings)]
 extern crate hyper;
 #[macro_use]

examples/echo.rs

@@ -1,4 +1,3 @@
-#![feature(async_await)]
 //#![deny(warnings)]
 
 use hyper::{Body, Method, Request, Response, Server, StatusCode};

examples/hello.rs

@@ -1,4 +1,3 @@
-#![feature(async_await)]
 #![deny(warnings)]
 
 use hyper::{Body, Request, Response, Server};

examples/multi_server.rs

@@ -1,4 +1,3 @@
-#![feature(async_await)]
 #![deny(warnings)]
 extern crate hyper;
 extern crate pretty_env_logger;

examples/params.rs

@@ -1,4 +1,3 @@
-#![feature(async_await)]
 // #![deny(warnings)]  // FIXME: https://github.com/rust-lang/rust/issues/62411
 extern crate hyper;
 extern crate pretty_env_logger;

examples/proxy.rs

@@ -1,4 +1,3 @@
-#![feature(async_await)]
 #![deny(warnings)]
 
 use hyper::{Client, Error, Server};

examples/send_file.rs

@@ -1,4 +1,3 @@
-#![feature(async_await)]
 #![deny(warnings)]
 
 use tokio::io::AsyncReadExt;

examples/single_threaded.rs

@@ -1,4 +1,3 @@
-#![feature(async_await)]
 #![deny(warnings)]
 
 use std::cell::Cell;

examples/state.rs

@@ -1,4 +1,3 @@
-#![feature(async_await)]
 #![deny(warnings)]
 
 use std::sync::{Arc, atomic::{AtomicUsize, Ordering}};

examples/tower_server.rs

@@ -1,4 +1,3 @@
-#![feature(async_await)]
 #![deny(warnings)]
 
 use hyper::{Body, Request, Response, Server};

examples/upgrades.rs

@@ -1,4 +1,3 @@
-#![feature(async_await)]
 #![deny(warnings)]
 
 // Note: `hyper::upgrade` docs link to this upgrade.

examples/web_api.rs

@@ -1,4 +1,3 @@
-#![feature(async_await)]
 #![deny(warnings)]
 
 use hyper::{Body, Chunk, Client, Method, Request, Response, Server, StatusCode, header};