remove mention of rust-lang#39364 from mpsc docs

Author: ibraheemdev

library/std/src/sync/mpsc/mod.rs

@@ -870,34 +870,6 @@ impl<T> Receiver<T> {
     /// However, since channels are buffered, messages sent before the disconnect
     /// will still be properly received.
     ///
-    /// # Known Issues
-    ///
-    /// There is currently a known issue (see [`#39364`]) that causes `recv_timeout`
-    /// to panic unexpectedly with the following example:
-    ///
-    /// ```no_run
-    /// use std::sync::mpsc::channel;
-    /// use std::thread;
-    /// use std::time::Duration;
-    ///
-    /// let (tx, rx) = channel::<String>();
-    ///
-    /// thread::spawn(move || {
-    ///     let d = Duration::from_millis(10);
-    ///     loop {
-    ///         println!("recv");
-    ///         let _r = rx.recv_timeout(d);
-    ///     }
-    /// });
-    ///
-    /// thread::sleep(Duration::from_millis(100));
-    /// let _c1 = tx.clone();
-    ///
-    /// thread::sleep(Duration::from_secs(1));
-    /// ```
-    ///
-    /// [`#39364`]: https://github.com/rust-lang/rust/issues/39364
-    ///
     /// # Examples
     ///
     /// Successfully receiving value before encountering timeout: