Remove dead futures2 code. (#538)

The futures 0.2 crate is not intended for widespread usage. Also, the
futures team is exploring the compat shim route.

If futures 0.3 support is added to Tokio 0.1, then a different
integration route will be explored, making the current code unhelpful.

tokio-threadpool/src/lib.rs

@@ -89,9 +89,6 @@ extern crate rand;
 #[macro_use]
 extern crate log;
 
-#[cfg(feature = "unstable-futures")]
-extern crate futures2;
-
 // ## Crate layout
 //
 // The primary type, `Pool`, holds the majority of a thread pool's state,
@@ -148,8 +145,6 @@ mod blocking;
 mod builder;
 mod callback;
 mod config;
-#[cfg(feature = "unstable-futures")]
-mod futures2_wake;
 mod notifier;
 mod pool;
 mod sender;