Mark loaderRequest __unused in RCTSyncImageManager to fix release-build unused-variable error (#57227)

Summary:
Pull Request resolved: #57227

Changelog: [Internal]

Reviewed By: ellemac123

Differential Revision: D108445102

fbshipit-source-id: 5bc01229668d233ca8d8260801dca3b0bada6e2d

Author: sammy-SC

packages/react-native/ReactCommon/react/renderer/imagemanager/platform/ios/react/renderer/imagemanager/RCTSyncImageManager.mm

@@ -78,7 +78,7 @@ - (ImageRequest)requestImage:(ImageSource)imageSource
     observerCoordinator->nativeImageResponseProgress((float)progress / (float)total, progress, total);
   };
 
-  RCTImageURLLoaderRequest *loaderRequest =
+  RCTImageURLLoaderRequest *__unused loaderRequest =
       [self->_imageLoader loadImageWithURLRequest:request
                                              size:CGSizeMake(imageSource.size.width, imageSource.size.height)
                                             scale:imageSource.scale