Fix thread parking on WebAssembly

Sleeping instead of parking with a timeout is not okay, even
on the WebAssembly target, which currently only supports
the current thread runtime.

Reasons:

1. A task may have registered a timer and also woken itself.
   Thus we need to continue execution immediately.

2. Even if Tokio only supports the current thread runtime on
  WebAssembly, there exist multi-threaded targets like
  wasm32-wasip1-threads. Thus a wake-up event can occur
  from another thread (outside the Tokio runtime).

tokio/src/runtime/park.rs

@@ -65,12 +65,7 @@ impl ParkThread {
     pub(crate) fn park_timeout(&mut self, duration: Duration) {
         #[cfg(loom)]
         CURRENT_THREAD_PARK_COUNT.with(|count| count.fetch_add(1, SeqCst));
-
-        // Wasm doesn't have threads, so just sleep.
-        #[cfg(not(target_family = "wasm"))]
         self.inner.park_timeout(duration);
-        #[cfg(target_family = "wasm")]
-        std::thread::sleep(duration);
     }
 
     pub(crate) fn shutdown(&mut self) {