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/enter.rs

@@ -3,9 +3,6 @@ use std::cell::Cell;
 use std::error::Error;
 use std::fmt;
 
-#[cfg(feature = "unstable-futures")]
-use futures2;
-
 thread_local!(static ENTERED: Cell<bool> = Cell::new(false));
 
 /// Represents an executor context.
@@ -14,9 +11,6 @@ thread_local!(static ENTERED: Cell<bool> = Cell::new(false));
 pub struct Enter {
     on_exit: Vec<Box<Callback>>,
     permanent: bool,
-
-    #[cfg(feature = "unstable-futures")]
-    _enter2: futures2::executor::Enter,
 }
 
 /// An error returned by `enter` if an execution scope has already been
@@ -66,9 +60,6 @@ pub fn enter() -> Result<Enter, EnterError> {
             Ok(Enter {
                 on_exit: Vec::new(),
                 permanent: false,
-
-                #[cfg(feature = "unstable-futures")]
-                _enter2: futures2::executor::enter().unwrap(),
             })
         }
     })