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/pool/mod.rs

@@ -116,9 +116,7 @@ impl Pool {
             backup_stack,
             blocking,
             shutdown_task: ShutdownTask {
-                task1: AtomicTask::new(),
-                #[cfg(feature = "unstable-futures")]
-                task2: futures2::task::AtomicWaker::new(),
+                task: AtomicTask::new(),
             },
             config,
         };