[ffigen] Remove objc_retain from bindings

[liamappelbe]

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[github-actions]

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Changelog Entry ❗

Package	Changed Files
package:cupertino_http	pkgs/cupertino_http/lib/src/native_cupertino_bindings.dart pkgs/cupertino_http/pubspec.yaml

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Coverage ⚠️

File	Coverage
pkgs/cupertino_http/lib/src/native_cupertino_bindings.dart	💔 Not covered

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License Headers ✔️

// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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Files
pkgs/http/example/main.dart
pkgs/http_client_conformance_tests/example/client_test.dart
pkgs/http_client_conformance_tests/lib/src/dummy_isolate.dart
pkgs/http_parser/test/example_test.dart
pkgs/http_profile/lib/http_profile.dart
pkgs/web_socket/example/web_socket_example.dart
pkgs/web_socket/lib/testing.dart
pkgs/web_socket_conformance_tests/example/client_test.dart