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-udp/src/lib.rs

@@ -29,9 +29,6 @@ extern crate tokio_codec;
 extern crate tokio_io;
 extern crate tokio_reactor;
 
-#[cfg(feature = "unstable-futures")]
-extern crate futures2;
-
 mod frame;
 mod socket;
 mod send_dgram;