chore: fix new clippy warnings (#1444)

This fixes a handful of new clippy lints. Should fix CI.

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

tracing-subscriber/src/layer.rs

@@ -90,8 +90,7 @@ use std::{any::TypeId, marker::PhantomData};
 ///
 /// Multiple `Layer`s may be composed in the same manner:
 /// ```rust
-/// # use tracing_subscriber::Layer;
-/// # use tracing_subscriber::prelude::*;
+/// # use tracing_subscriber::{Layer, SubscriberExt};
 /// # use tracing::Subscriber;
 /// pub struct MyOtherLayer {
 ///     // ...