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-reactor/src/background.rs

@@ -1,4 +1,4 @@
-use {Reactor, Handle, Task};
+use {Reactor, Handle};
 use atomic_task::AtomicTask;
 
 use futures::{Future, Async, Poll, task};
@@ -136,7 +136,7 @@ impl Future for Shutdown {
     type Error = ();
 
     fn poll(&mut self) -> Poll<(), ()> {
-        let task = Task::Futures1(task::current());
+        let task = task::current();
         self.inner.shared.shutdown_task.register_task(task);
 
         if !self.inner.is_shutdown() {