Remove #[rustc_macro_edition_2021].

It was only temporarily used by pin!(), which no longer needs it.

compiler/rustc_attr_parsing/src/attributes/mod.rs

@@ -28,7 +28,6 @@ pub(crate) mod cfg;
 pub(crate) mod confusables;
 pub(crate) mod deprecation;
 pub(crate) mod repr;
-pub(crate) mod rustc;
 pub(crate) mod stability;
 pub(crate) mod transparency;
 pub(crate) mod util;