Remove the unstable and deprecated mpsc_select

This removes macro `select!` and `std::sync::mpsc::{Handle, Select}`,
which were all unstable and have been deprecated since 1.32.

src/test/run-pass/issues/issue-13494.rs

@@ -1,35 +0,0 @@
-// run-pass
-#![allow(unused_must_use)]
-// ignore-emscripten no threads support
-
-// This test may not always fail, but it can be flaky if the race it used to
-// expose is still present.
-
-#![feature(mpsc_select)]
-#![allow(deprecated)]
-
-use std::sync::mpsc::{channel, Sender, Receiver};
-use std::thread;
-
-fn helper(rx: Receiver<Sender<()>>) {
-    for tx in rx.iter() {
-        let _ = tx.send(());
-    }
-}
-
-fn main() {
-    let (tx, rx) = channel();
-    let t = thread::spawn(move|| { helper(rx) });
-    let (snd, rcv) = channel::<isize>();
-    for _ in 1..100000 {
-        snd.send(1).unwrap();
-        let (tx2, rx2) = channel();
-        tx.send(tx2).unwrap();
-        select! {
-            _ = rx2.recv() => (),
-            _ = rcv.recv() => ()
-        }
-    }
-    drop(tx);
-    t.join();
-}

src/test/run-pass/macros/macro-comma-support.rs

@@ -233,8 +233,6 @@ fn println() {
     println!("hello {}", "world",);
 }
 
-// select! is too troublesome and unlikely to be stabilized
-
 // stringify! is N/A
 
 #[cfg(std)]