Remove unnecessary mut.

The value is moved in pin!().

tests/ui/sanitizer/cfi/coroutine.rs

@@ -26,7 +26,7 @@ use std::async_iter::AsyncIterator;
 
 #[test]
 fn general_coroutine() {
-    let mut coro = #[coroutine] |x: i32| {
+    let coro = #[coroutine] |x: i32| {
         yield x;
         "done"
     };