Make futures::task::noop_waker_ref available without std. (#2505)

Commit 232ed5f removed the std
dependency, and 4c09f82 made it
exported unconditionally from `futures_task`, but the `cfg` on the
reexport from `futures_util` (and hence indirectly `futures`) was left
in place until now.

futures-util/src/task/mod.rs

@@ -16,7 +16,6 @@ pub use core::task::{Context, Poll, RawWaker, RawWakerVTable, Waker};
 pub use futures_task::{FutureObj, LocalFutureObj, LocalSpawn, Spawn, SpawnError, UnsafeFutureObj};
 
 pub use futures_task::noop_waker;
-#[cfg(feature = "std")]
 pub use futures_task::noop_waker_ref;
 
 #[cfg(not(futures_no_atomic_cas))]