[WIP] 依存管理にはSwift Package Manager を利用するので Podfile を削除する

[zztkm]

• [UPDATE] 依存管理を CocoaPods から Swift Package Manager に移行したため Podfile を削除する

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This pull request includes several changes related to dependency management and build configuration for the project. The most important changes include the removal of the Podfile and the addition of a new Makefile for running formatters and linters.

Dependency management changes:

• CHANGES.md: Updated to reflect the migration from CocoaPods to Swift Package Manager and the removal of the Podfile.
• Podfile: Completely removed as part of the transition to Swift Package Manager.

Build configuration changes:

• CHANGES.md: Updated to include the addition of a new Makefile for running swift-format and SwiftLint, replacing the old lint-format.sh script.

[zztkm]

CI 周りを xcodebuild から SwiftPM に移行するのを先にやった方が変更を小さくできて良さそうだなと思い直したので、Podfile 削除は CI 整備より後で対応しようと思います。この PR は一度クローズします。