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-current-thread/tests/current_thread.rs

@@ -1,5 +1,3 @@
-#![cfg(not(feature = "unstable-futures"))]
-
 extern crate tokio_current_thread;
 extern crate tokio_executor;
 extern crate futures;