Reduce worker_manager memory allocation (#4435)

ios/Runner/AppDelegate.swift

@@ -1,6 +1,5 @@
 import UIKit
 import Flutter
-import flutter_downloader
 import receive_sharing_intent
 import flutter_local_notifications
 
@@ -34,12 +33,6 @@ import flutter_local_notifications
             GeneratedPluginRegistrant.register(with: registry)
         }
         
-        FlutterDownloaderPlugin.setPluginRegistrantCallback { registry in
-            if (!registry.hasPlugin("FlutterDownloaderPlugin")) {
-                FlutterDownloaderPlugin.register(with: registry.registrar(forPlugin: "FlutterDownloaderPlugin")!)
-            }
-        }
-        
         let sharingIntent = SwiftReceiveSharingIntentPlugin.instance
         if let url = launchOptions?[UIApplication.LaunchOptionsKey.url] as? URL {
             if url.scheme == "mailto" {