Don't set builtin_name for builtin macro implementations.

This used to be necessary for `is_builtin` in the past, but is no longer required.

Co-authored-by: Vadim Petrochenkov <vadim.petrochenkov@gmail.com>

compiler/rustc_builtin_macros/src/lib.rs

@@ -48,7 +48,7 @@ pub fn register_builtin_macros(resolver: &mut dyn ResolverExpand, edition: Editi
     let mut register = |name, kind| {
         resolver.register_builtin_macro(
             Ident::with_dummy_span(name),
-            SyntaxExtension { builtin_name: Some(name), ..SyntaxExtension::default(kind, edition) },
+            SyntaxExtension::default(kind, edition),
         )
     };
     macro register_bang($($name:ident: $f:expr,)*) {