chore: Fix new nightly warnings

Signed-off-by: John Nunley <dev@notgull.net>

benches/executor.rs

@@ -1,4 +1,3 @@
-use std::future::Future;
 use std::thread::available_parallelism;
 
 use async_executor::Executor;

examples/priority.rs

@@ -1,6 +1,5 @@
 //! An executor with task priorities.
 
-use std::future::Future;
 use std::thread;
 
 use async_executor::{Executor, Task};

src/lib.rs

@@ -34,7 +34,6 @@
 )]
 
 use std::fmt;
-use std::future::Future;
 use std::marker::PhantomData;
 use std::panic::{RefUnwindSafe, UnwindSafe};
 use std::rc::Rc;