Fix clippy::redundant_async_block warning

```
warning: this async expression only awaits a single future
  --> futures-util/benches/select.rs:21:30
   |
21 |           let count = block_on(async {
   |  ______________________________^
22 | |             select(
23 | |                 stream1,
24 | |                 select(
...  |
30 | |             .await
31 | |         });
   | |_________^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_async_block
   = note: `#[warn(clippy::redundant_async_block)]` on by default
help: you can reduce it to
   |
21 ~         let count = block_on(select(
22 +                 stream1,
23 +                 select(
24 +                     stream2,
25 +                     select(stream3, select(stream4, select(stream5, select(stream6, stream7)))),
26 +                 ),
27 +             )
28 ~             .count());
   |

warning: this async expression only awaits a single future
  --> futures/tests/compat.rs:12:13
   |
12 |     let f = async { Delay::new(Instant::now()).compat().await };
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: you can reduce it to: `Delay::new(Instant::now()).compat()`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_async_block
   = note: `#[warn(clippy::redundant_async_block)]` on by default
```

Author: taiki-e

futures-util/benches/select.rs

@@ -18,17 +18,16 @@ fn select_streams(b: &mut Bencher) {
         let stream5 = repeat(5).take(STREAM_COUNT);
         let stream6 = repeat(6).take(STREAM_COUNT);
         let stream7 = repeat(7).take(STREAM_COUNT);
-        let count = block_on(async {
+        let count = block_on(
             select(
                 stream1,
                 select(
                     stream2,
                     select(stream3, select(stream4, select(stream5, select(stream6, stream7)))),
                 ),
             )
-            .count()
-            .await
-        });
+            .count(),
+        );
         assert_eq!(count, STREAM_COUNT * 7);
     });
 }

futures/tests/compat.rs

@@ -9,7 +9,7 @@ use tokio::timer::Delay;
 
 #[test]
 fn can_use_01_futures_in_a_03_future_running_on_a_01_executor() {
-    let f = async { Delay::new(Instant::now()).compat().await };
+    let f = Delay::new(Instant::now()).compat();
 
     let mut runtime = Runtime::new().unwrap();
     runtime.block_on(f.boxed().compat()).unwrap();