core: derive Default for NoSubscriber (tokio-rs#1785)

* core: derive `Default` for `NoSubscriber`
* additional clippy nonsense

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tracing-core/src/subscriber.rs

@@ -568,15 +568,9 @@ impl Interest {
 ///
 /// [`NoSubscriber`] implements the [`Subscriber`] trait by never being enabled,
 /// never being interested in any callsite, and dropping all spans and events.
-#[derive(Debug, Copy, Clone)]
+#[derive(Copy, Clone, Debug, Default)]
 pub struct NoSubscriber(());
 
-impl Default for NoSubscriber {
- fn default() -> Self {
- NoSubscriber(())
- }
-}
-
 impl Subscriber for NoSubscriber {
  #[inline]
  fn register_callsite(&self, _: &'static Metadata<'static>) -> Interest {