Only base async fn arity on non-default/non-rest params - fixes #4891 (#4901)

packages/babel-plugin-transform-async-generator-functions/test/fixtures/nested/async-in-params/expected.js

@@ -6,7 +6,7 @@ let g = (() => {
     yield 3;
   });
 
-  return function g(_x) {
+  return function g() {
     return _ref.apply(this, arguments);
   };
 })();