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/global.rs

@@ -4,9 +4,6 @@ use futures::Future;
 
 use std::cell::Cell;
 
-#[cfg(feature = "unstable-futures")]
-use futures2;
-
 /// Executes futures on the default executor for the current execution context.
 ///
 /// `DefaultExecutor` implements `Executor` and can be used to spawn futures
@@ -65,7 +62,7 @@ enum State {
     Ready(*mut Executor),
     // default executor is currently active (used to detect recursive calls)
     Active
-} 
+}
 
 /// Thread-local tracking the current executor
 thread_local!(static EXECUTOR: Cell<State> = Cell::new(State::Empty));
@@ -80,14 +77,6 @@ impl super::Executor for DefaultExecutor {
             .unwrap_or_else(|| Err(SpawnError::shutdown()))
     }
 
-    #[cfg(feature = "unstable-futures")]
-    fn spawn2(&mut self, future: Box<futures2::Future<Item = (), Error = futures2::Never> + Send>)
-             -> Result<(), futures2::executor::SpawnError>
-    {
-        DefaultExecutor::with_current(|executor| executor.spawn2(future))
-            .unwrap_or_else(|| Err(futures2::executor::SpawnError::shutdown()))
-    }
-
     fn status(&self) -> Result<(), SpawnError> {
         DefaultExecutor::with_current(|executor| executor.status())
             .unwrap_or_else(|| Err(SpawnError::shutdown()))
@@ -142,15 +131,6 @@ pub fn spawn<T>(future: T)
         .unwrap()
 }
 
-/// Like `spawn` but compatible with futures 0.2
-#[cfg(feature = "unstable-futures")]
-pub fn spawn2<T>(future: T)
-    where T: futures2::Future<Item = (), Error = futures2::Never> + Send + 'static,
-{
-    DefaultExecutor::current().spawn2(Box::new(future))
-        .unwrap()
-}
-
 /// Set the default executor for the duration of the closure
 ///
 /// # Panics