simplify some pattern matches to appease 1.68 clippy (#13833)

substrate/frame/session/src/lib.rs

@@ -678,7 +678,7 @@ impl<T: Config> Pallet<T> {
 				// since a new validator set always leads to `changed` starting
 				// as true, we can ensure that `now_session_keys` and `next_validators`
 				// have the same length. this function is called once per iteration.
-				if let Some(&(_, ref old_keys)) = now_session_keys.next() {
+				if let Some((_, old_keys)) = now_session_keys.next() {
 					if old_keys != keys {
 						changed = true;
 					}