Remove old `GitGarbageCollectWorker` worker

In !52064 (merged) we namespaced the GitGarbageCollectWorker worker into Projects::GitGarbageCollectWorker. Nevertheless, at that time we couldn't get rid of the original worker because, according to our guidelines, we can only remove workers on major releases (https://docs.gitlab.com/ee/development/sidekiq_style_guide.html#removing-workers), so we had to wait for %14.0.

We can now remove the old worker.

At the same time, we're enforcing now classes to be namespaced (!51236 (merged)). Therefore, once we remove the old worker, we can also remove the path from the list of excluded files (https://gitlab.com/gitlab-org/gitlab/-/blob/24c7171d3939b5be1ccc14755b7dfbb58f3da85a/.rubocop_manual_todo.yml#L2338).