refactor(iOS): simplify UINavigationControllerDelegate methods implem…

…entation (#2562)

## Description

Noticed a low-hanging opportunity to simplify this method while doing
something different.
We already had this loop implemented in helper function so we could use
it.

## Test code and steps to reproduce

CI should be enough

## Checklist

- [ ] Ensured that CI passes

ios/RNSScreenStack.mm

@@ -244,21 +244,16 @@ - (void)navigationController:(UINavigationController *)navigationController
       willShowViewController:(UIViewController *)viewController
                     animated:(BOOL)animated
 {
-  UIView *view = viewController.view;
 #ifdef RCT_NEW_ARCH_ENABLED
-  if (![view isKindOfClass:[RNSScreenView class]]) {
+  if (![viewController.view isKindOfClass:[RNSScreenView class]]) {
     // if the current view is a snapshot, config was already removed so we don't trigger the method
     return;
   }
 #endif
-  RNSScreenStackHeaderConfig *config = nil;
-  for (UIView *subview in view.reactSubviews) {
-    if ([subview isKindOfClass:[RNSScreenStackHeaderConfig class]]) {
-      config = (RNSScreenStackHeaderConfig *)subview;
-      break;
-    }
-  }
-  [RNSScreenStackHeaderConfig willShowViewController:viewController animated:animated withConfig:config];
+  auto *screenView = static_cast<RNSScreenView *>(viewController.view);
+  [RNSScreenStackHeaderConfig willShowViewController:viewController
+                                            animated:animated
+                                          withConfig:screenView.findHeaderConfig];
 }
 
 - (void)presentationControllerDidDismiss:(UIPresentationController *)presentationController