Remove #![feature(async_await)] from examples

examples/hello_world.rs

@@ -1,5 +1,3 @@
-#![feature(async_await)]
-
 use std::{convert::TryFrom, env, process::exit};
 
 use ruma_client::{

examples/message_log.rs

@@ -1,5 +1,3 @@
-#![feature(async_await)]
-
 use std::{env, process::exit};
 
 use futures::stream::{StreamExt as _, TryStreamExt as _};