ignore #[rustc_allow_const_fn_unstable] for macro expansion

Recognition for `rustc_allow_const_fn_unstable` attribute was errorneously
added in 05f4a9a.

Author: liketechnik

compiler/rustc_expand/src/base.rs

@@ -768,13 +768,8 @@ impl SyntaxExtension {
         name: Symbol,
         attrs: &[ast::Attribute],
     ) -> SyntaxExtension {
-        let allow_internal_unstable = {
-            let mut feat_list = Vec::new();
-            attr::allow_internal_unstable(sess, &attrs).map(|features| feat_list.extend(features));
-            attr::rustc_allow_const_fn_unstable(sess, &attrs)
-                .map(|features| feat_list.extend(features));
-            Some(feat_list.into())
-        };
+        let allow_internal_unstable = attr::allow_internal_unstable(sess, &attrs)
+            .map(|features| features.collect::<Vec<Symbol>>().into());
 
         let mut local_inner_macros = false;
         if let Some(macro_export) = sess.find_by_name(attrs, sym::macro_export) {