subscriber: remove Layer impls for Arcs (#1649)

Implementing `Layer` for `Arc`s, which are immutable, breaks the
ability to implement `Layer::on_layer` with a mutable reference.
This is necessary for per-layer filtering. See
#1576 (comment) for
details. Therefore, the `Layer` impls for `Arc`s should not be used.

In 0.3, we have the opportunity to remove these APIs. Therefore, this PR
removes them.

tracing-subscriber/src/subscribe/mod.rs

@@ -417,7 +417,11 @@
 //! [`LevelFilter`]: crate::filter::LevelFilter
 //! [feat]: crate#feature-flags
 use crate::filter;
-use std::{any::TypeId, ops::Deref, ptr::NonNull, sync::Arc};
+use std::{
+    any::TypeId,
+    ops::{Deref, DerefMut},
+    ptr::NonNull,
+};
 use tracing_core::{
     collect::{Collect, Interest},
     metadata::Metadata,
@@ -1083,6 +1087,11 @@ where
 
 macro_rules! layer_impl_body {
     () => {
+        #[inline]
+        fn on_subscribe(&mut self, collect: &mut C) {
+            self.deref_mut().on_subscribe(collect);
+        }
+
         #[inline]
         fn on_new_span(&self, attrs: &span::Attributes<'_>, id: &span::Id, ctx: Context<'_, C>) {
             self.deref().on_new_span(attrs, id, ctx)
@@ -1146,21 +1155,6 @@ macro_rules! layer_impl_body {
     };
 }
 
-impl<S, C> Subscribe<C> for Arc<S>
-where
-    S: Subscribe<C>,
-    C: Collect,
-{
-    layer_impl_body! {}
-}
-
-impl<C> Subscribe<C> for Arc<dyn Subscribe<C>>
-where
-    C: Collect,
-{
-    layer_impl_body! {}
-}
-
 impl<S, C> Subscribe<C> for Box<S>
 where
     S: Subscribe<C>,