attributes: fix clippy warning in attributes tests (#2742)

Clippy doesn't like the redefinition of a binding with itself. I don't
think this was necessary for the bug we were reproducing here (as the
error test doesn't do this), so I removed it.

tracing-attributes/tests/async_fn.rs

@@ -17,7 +17,6 @@ async fn test_async_fn(polls: usize) -> Result<(), ()> {
 #[allow(dead_code)] // this is just here to test whether it compiles.
 #[instrument]
 async fn test_ret_impl_trait(n: i32) -> Result<impl Iterator<Item = i32>, ()> {
-    let n = n;
     Ok((0..10).filter(move |x| *x < n))
 }