Remove #[rustc_macro_edition_2021].

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

compiler/rustc_attr_parsing/src/context.rs

@@ -15,7 +15,6 @@ use crate::attributes::allow_unstable::{AllowConstFnUnstableParser, AllowInterna
 use crate::attributes::confusables::ConfusablesParser;
 use crate::attributes::deprecation::DeprecationParser;
 use crate::attributes::repr::ReprParser;
-use crate::attributes::rustc::RustcMacroEdition2021Parser;
 use crate::attributes::stability::{
     BodyStabilityParser, ConstStabilityIndirectParser, ConstStabilityParser, StabilityParser,
 };
@@ -77,7 +76,6 @@ attribute_groups!(
         // tidy-alphabetical-start
         Single<ConstStabilityIndirectParser>,
         Single<DeprecationParser>,
-        Single<RustcMacroEdition2021Parser>,
         Single<TransparencyParser>,
         // tidy-alphabetical-end
     ];