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-executor/src/lib.rs

@@ -37,9 +37,6 @@
 
 extern crate futures;
 
-#[cfg(feature = "unstable-futures")]
-extern crate futures2;
-
 mod enter;
 mod global;
 pub mod park;
@@ -47,9 +44,6 @@ pub mod park;
 pub use enter::{enter, Enter, EnterError};
 pub use global::{spawn, with_default, DefaultExecutor};
 
-#[cfg(feature = "unstable-futures")]
-pub use global::spawn2;
-
 use futures::Future;
 
 use std::error::Error;
@@ -142,11 +136,6 @@ pub trait Executor {
     fn spawn(&mut self, future: Box<Future<Item = (), Error = ()> + Send>)
              -> Result<(), SpawnError>;
 
-    /// Like `spawn`, but compatible with futures 0.2
-    #[cfg(feature = "unstable-futures")]
-    fn spawn2(&mut self, future: Box<futures2::Future<Item = (), Error = futures2::Never> + Send>)
-             -> Result<(), futures2::executor::SpawnError>;
-
     /// Provides a best effort **hint** to whether or not `spawn` will succeed.
     ///
     /// This function may return both false positives **and** false negatives.
@@ -194,13 +183,6 @@ impl<E: Executor + ?Sized> Executor for Box<E> {
         (**self).spawn(future)
     }
 
-    #[cfg(feature = "unstable-futures")]
-    fn spawn2(&mut self, future: Box<futures2::Future<Item = (), Error = futures2::Never> + Send>)
-        -> Result<(), futures2::executor::SpawnError>
-    {
-        (**self).spawn2(future)
-    }
-
     fn status(&self) -> Result<(), SpawnError> {
         (**self).status()
     }