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/shutdown_task.rs

@@ -1,24 +1,12 @@
 use futures::task::AtomicTask;
-#[cfg(feature = "unstable-futures")]
-use futures2;
 
 #[derive(Debug)]
 pub(crate) struct ShutdownTask {
-    pub task1: AtomicTask,
-
-    #[cfg(feature = "unstable-futures")]
-    pub task2: futures2::task::AtomicWaker,
+    pub task: AtomicTask,
 }
 
 impl ShutdownTask {
-    #[cfg(not(feature = "unstable-futures"))]
     pub fn notify(&self) {
-        self.task1.notify();
-    }
-
-    #[cfg(feature = "unstable-futures")]
-    pub fn notify(&self) {
-        self.task1.notify();
-        self.task2.wake();
+        self.task.notify();
     }
 }