Fix clippy::single_match warning

```
error: you seem to be trying to use `match` for destructuring a single pattern. Consider using `if let`
   --> futures-executor/src/thread_pool.rs:349:9
    |
349 | /         match arc_self.mutex.notify() {
350 | |             Ok(task) => arc_self.exec.state.send(Message::Run(task)),
351 | |             Err(()) => {}
352 | |         }
    | |_________^ help: try this: `if let Ok(task) = arc_self.mutex.notify() { arc_self.exec.state.send(Message::Run(task)) }`
    |
    = note: `-D clippy::single-match` implied by `-D warnings`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#single_match
```

futures-executor/src/thread_pool.rs

@@ -346,9 +346,8 @@ impl fmt::Debug for Task {
 
 impl ArcWake for WakeHandle {
     fn wake_by_ref(arc_self: &Arc<Self>) {
-        match arc_self.mutex.notify() {
-            Ok(task) => arc_self.exec.state.send(Message::Run(task)),
-            Err(()) => {}
+        if let Ok(task) = arc_self.mutex.notify() {
+            arc_self.exec.state.send(Message::Run(task))
         }
     }
 }