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.

src/executor/mod.rs

@@ -61,9 +61,6 @@ pub use tokio_executor::{Executor, DefaultExecutor, SpawnError};
 use futures::{Future, IntoFuture};
 use futures::future::{self, FutureResult};
 
-#[cfg(feature = "unstable-futures")]
-use futures2;
-
 /// Return value from the `spawn` function.
 ///
 /// Currently this value doesn't actually provide any functionality. However, it
@@ -133,15 +130,6 @@ where F: Future<Item = (), Error = ()> + 'static + Send
     Spawn(())
 }
 
-/// Like `spawn`, but compatible with futures 0.2
-#[cfg(feature = "unstable-futures")]
-pub fn spawn2<F>(f: F) -> Spawn
-    where F: futures2::Future<Item = (), Error = futures2::Never> + 'static + Send
-{
-    ::tokio_executor::spawn2(f);
-    Spawn(())
-}
-
 impl IntoFuture for Spawn {
     type Future = FutureResult<(), ()>;
     type Item = ();
@@ -151,14 +139,3 @@ impl IntoFuture for Spawn {
         future::ok(())
     }
 }
-
-#[cfg(feature = "unstable-futures")]
-impl futures2::IntoFuture for Spawn {
-    type Future = futures2::future::FutureResult<(), ()>;
-    type Item = ();
-    type Error = ();
-
-    fn into_future(self) -> Self::Future {
-        futures2::future::ok(())
-    }
-}