subscriber: revert "impl LookupSpan for Box<LS> and Arc<LS> (#2247)"

This reverts commit a0824d3 (PR #2247).
As discussed in [this comment][1], the implementation for `Arc`s may
cause subtly incorrect behavior if actually used, due to the `&mut self`
receiver of the `LookupSpan::register_filter` method, since the `Arc`
cannot be mutably borrowed if any clones of it exist.

The APIs added in PRs #2269 and #2293 offer an alternative solution to
the same problems this change was intended to solve, and --- since this
change hasn't been published yet --- it can safely be reverted.

[1]:
    https://giethub.com/tokio-rs/tracing/pull/2247#issuecomment-1199924876

Author: hawkw

tracing-subscriber/src/layer/mod.rs

@@ -685,7 +685,7 @@ use core::any::TypeId;
 
 feature! {
     #![feature = "alloc"]
-    use alloc::{vec::Vec, boxed::Box};
+    use alloc::boxed::Box;
     use core::ops::{Deref, DerefMut};
 }
 
@@ -1656,6 +1656,8 @@ where
 
 feature! {
     #![any(feature = "std", feature = "alloc")]
+    #[cfg(not(feature = "std"))]
+    use alloc::vec::Vec;
 
     macro_rules! layer_impl_body {
         () => {

tracing-subscriber/src/registry/mod.rs

@@ -230,11 +230,6 @@ pub struct Scope<'a, R> {
 feature! {
     #![any(feature = "alloc", feature = "std")]
 
-    use alloc::{
-        boxed::Box,
-        sync::Arc
-    };
-
     #[cfg(not(feature = "smallvec"))]
     use alloc::vec::{self, Vec};
 
@@ -256,66 +251,6 @@ feature! {
     #[cfg(feature = "smallvec")]
     type SpanRefVecArray<'span, L> = [SpanRef<'span, L>; 16];
 
-    impl<'a, S> LookupSpan<'a> for Arc<S>
-    where
-        S: LookupSpan<'a>,
-    {
-        type Data = <S as LookupSpan<'a>>::Data;
-
-        fn span_data(&'a self, id: &Id) -> Option<Self::Data> {
-            self.as_ref().span_data(id)
-        }
-
-        fn span(&'a self, id: &Id) -> Option<SpanRef<'_, Self>>
-        where
-            Self: Sized,
-        {
-            self.as_ref().span(id).map(
-                |SpanRef {
-                    registry: _,
-                    data,
-                    #[cfg(feature = "registry")]
-                    filter,
-                }| SpanRef {
-                    registry: self,
-                    data,
-                    #[cfg(feature = "registry")]
-                    filter,
-                },
-            )
-        }
-    }
-
-    impl<'a, S> LookupSpan<'a> for Box<S>
-    where
-        S: LookupSpan<'a>,
-    {
-        type Data = <S as LookupSpan<'a>>::Data;
-
-        fn span_data(&'a self, id: &Id) -> Option<Self::Data> {
-            self.as_ref().span_data(id)
-        }
-
-        fn span(&'a self, id: &Id) -> Option<SpanRef<'_, Self>>
-        where
-            Self: Sized,
-        {
-            self.as_ref().span(id).map(
-                |SpanRef {
-                    registry: _,
-                    data,
-                    #[cfg(feature = "registry")]
-                    filter,
-                }| SpanRef {
-                    registry: self,
-                    data,
-                    #[cfg(feature = "registry")]
-                    filter,
-                },
-            )
-        }
-    }
-
     impl<'a, R> Scope<'a, R>
     where
         R: LookupSpan<'a>,