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-threadpool/src/builder.rs

@@ -16,9 +16,6 @@ use num_cpus;
 use tokio_executor::Enter;
 use tokio_executor::park::Park;
 
-#[cfg(feature = "unstable-futures")]
-use futures2;
-
 /// Builds a thread pool with custom configuration values.
 ///
 /// Methods can be chained in order to set the configuration values. The thread