Fix warnings about unnecessary 'use' due to Box, Send and Sync being defined in the language prelude.

PiperOrigin-RevId: 615533149

rust/cpp.rs

@@ -727,7 +727,6 @@ impl_ProxiedInMapValue_for_key_types!(
 mod tests {
     use super::*;
     use googletest::prelude::*;
-    use std::boxed::Box;
 
     // We need to allocate the byte array so SerializedData can own it and
     // deallocate it in its drop. This function makes it easier to do so for our

rust/proxied.rs

@@ -48,7 +48,6 @@ use crate::RepeatedMut;
 use crate::__internal::Private;
 use crate::repeated::ProxiedInRepeated;
 use std::fmt::Debug;
-use std::marker::{Send, Sync};
 
 /// A type that can be accessed through a reference-like proxy.
 ///