async-await: track nightly changes (#661)

The `tokio-async-await` crate is no longer a facade. Instead, the `tokio` crate
provides a feature flag to enable async/await support.

tokio-reactor/src/async_await.rs

@@ -1,5 +0,0 @@
-use Registration;
-
-use std::marker::Unpin;
-
-impl Unpin for Registration {}

tokio-reactor/src/lib.rs

@@ -1,6 +1,5 @@
 #![doc(html_root_url = "https://docs.rs/tokio-reactor/0.1.5")]
 #![deny(missing_docs, warnings, missing_debug_implementations)]
-#![cfg_attr(feature = "async-await-preview", feature(pin))]
 
 //! Event loop that drives Tokio I/O resources.
 //!
@@ -757,8 +756,3 @@ impl Error for SetFallbackError {
         "attempted to set fallback reactor while already configured"
     }
 }
-
-// ===== EXPERIMENTAL async / await support =====
-
-#[cfg(feature = "async-await-preview")]
-mod async_await;