fix false must_use warning on JoinHandle

Removes the must_use requirement on the JoinHandle, since it is the
result of a future being spawned - thus, the originating future will be
polled to completion, whether or not the JoinHandle is polled.

Author: rschmukler

src/task.rs

@@ -85,7 +85,6 @@ where
 /// A handle that awaits the result of a [`spawn`]ed future.
 ///
 /// [`spawn`]: fn.spawn.html
-#[must_use = "futures do nothing unless you `.await` or poll them"]
 #[derive(Debug)]
 pub struct JoinHandle<T> {
     pub(crate) rx: futures::channel::oneshot::Receiver<T>,