Fix warnings in benches

Author: kpp

benches/future.rs

@@ -1,9 +1,8 @@
-#![feature(async_await)]
 #![feature(async_closure)]
 
 #[macro_use]
 extern crate criterion;
-use criterion::{Criterion, Benchmark, BatchSize, ParameterizedBenchmark, black_box};
+use criterion::{Criterion, Benchmark, black_box};
 
 use futures::executor;
 

benches/stream.rs

@@ -1,5 +1,3 @@
-#![feature(async_await)]
-
 #[macro_use]
 extern crate criterion;
 use criterion::{Criterion, ParameterizedBenchmark};