Remove code used for third-party container registries

Context

Support for third-party container registry has been deprecated and removed in %16.0 (#376217 (closed)).

Task

We have two special services used for cleaning up tags in third-party container registries in Projects::ContainerRepository::ThirdParty, namely CleanupTagsService and DeleteTagsService. Given that we no longer support third-party registries we can remove this and update all callers to use the default implementations in Projects::ContainerRepository::Gitlab.

While at it, we can also refactor the code by relocating both services from Projects::ContainerRepository::Gitlab into Projects::ContainerRepository, as it's no longer necessary to distinguish between Gitlab and ThirdParty registries.