Remove plugin_platform_interface

[natsuk4ze]

Until now, gal had postponed adopting a federated plugin implementation, as a substantial redesign would be necessary if adopted. However, with Dart 3's base modifier, this package has become unnecessary except when creating mocks. In Gal's testing, the integration tests completely cover the code without the need for mocks. Therefore, the plugin_platform_interface package has been removed.

see: a-note-about-base