stream: remove unreachable code

isIterable() is called in one place with only one argument, so the
second argument and associated logic for it can be removed.

lib/stream/promises.js

@@ -30,10 +30,8 @@ function isStream(obj) {
   return isReadable(obj) || isWritable(obj);
 }
 
-function isIterable(obj, isAsync) {
+function isIterable(obj) {
   if (!obj) return false;
-  if (isAsync === true) return typeof obj[SymbolAsyncIterator] === 'function';
-  if (isAsync === false) return typeof obj[SymbolIterator] === 'function';
   return typeof obj[SymbolAsyncIterator] === 'function' ||
     typeof obj[SymbolIterator] === 'function';
 }