style: Avoid some allocations in selector serialization.

The allocations in display_to_css_identifier show up in the profiles of
bug 1675628.

Differential Revision: https://phabricator.services.mozilla.com/D97856

components/style/gecko_string_cache/mod.rs

@@ -42,12 +42,6 @@ pub mod namespace;
 
 pub use self::namespace::{Namespace, WeakNamespace};
 
-macro_rules! local_name {
-    ($s:tt) => {
-        atom!($s)
-    };
-}
-
 /// A handle to a Gecko atom. This is a type that can represent either:
 ///
 ///  * A strong reference to a dynamic atom (an `nsAtom` pointer), in which case