[iOS] Make FlutterEngine new method available (#25446)

shell/platform/darwin/ios/framework/Headers/FlutterEngine.h

@@ -161,8 +161,6 @@ FLUTTER_DARWIN_EXPORT
       allowHeadlessExecution:(BOOL)allowHeadlessExecution
           restorationEnabled:(BOOL)restorationEnabled NS_DESIGNATED_INITIALIZER;
 
-+ (instancetype)new NS_UNAVAILABLE;
-
 /**
  * Runs a Dart program on an Isolate from the main Dart library (i.e. the library that
  * contains `main()`), using `main()` as the entrypoint (the default for Flutter projects),