Bug 1918093: Convert servo_arc to edition 2021 (r=emilio)

Signed-off-by: Nico Burns <nico@nicoburns.com>

Differential Revision: https://phabricator.services.mozilla.com/D221818

servo/components/servo_arc/Cargo.toml

@@ -5,6 +5,7 @@ authors = ["The Servo Project Developers"]
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/servo/servo"
 description = "A fork of std::sync::Arc with some extra functionality and without weak references"
+edition = "2021"
 
 [lib]
 name = "servo_arc"

servo/components/servo_arc/lib.rs

@@ -25,10 +25,6 @@
 // duplicate those here.
 #![allow(missing_docs)]
 
-#[cfg(feature = "servo")]
-extern crate serde;
-extern crate stable_deref_trait;
-
 #[cfg(feature = "servo")]
 use serde::{Deserialize, Serialize};
 use stable_deref_trait::{CloneStableDeref, StableDeref};