Remove `Projects::HousekeepingService`

In !51572 (merged) we're moving the Projects::HousekeepingService service outside of the Projects namespace in order to make it more generic and reuse it with group and project wikis.

Nevertheless, in that MR we couldn't get rid of the class because, during deployment, there can be sidekiq jobs referencing the old class and we don't want exceptions to come. Instead, since it was merged in 13.9, we're creating this issue for 13.10, in order to ensure that no sidekiq job is still referencing the old class.

We need to remove as well the spec file because we still needed to ensure that the old class behaves as before.