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

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

tests/client.rs

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

tests/server.rs

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