chore(utils): refactored isAsyncFn util to avoid inlining additional …

…Babel helpers to the build; (axios#5684)

lib/utils.js

@@ -662,11 +662,7 @@ const toJSONObject = (obj) => {
   return visit(obj, 0);
 }
 
-const [isPlainFunction, isAsyncFn, isGeneratorFn, isAsyncGeneratorFn] = (
-  (...fns) => fns.map(
-    ({constructor})=> (thing) => thing && typeof thing === 'function' && thing.constructor === constructor
-  )
-)(()=> {}, async()=>{}, function*(){}, async function*(){});
+const isAsyncFn = kindOfTest('AsyncFunction');
 
 const isThenable = (thing) =>
   thing && (isObject(thing) || isFunction(thing)) && isFunction(thing.then) && isFunction(thing.catch);
@@ -722,8 +718,5 @@ export default {
   isSpecCompliantForm,
   toJSONObject,
   isAsyncFn,
-  isGeneratorFn,
-  isAsyncGeneratorFn,
-  isPlainFunction,
   isThenable
 };