destroy_service.rb

1. When removing projects, we can end-up leaving the +deleted
repo path dirty and not successfully removing the non-deleted
namespace (mv process is not atomic and can be killed without
fully moving the path).

2. In order to solve that, we're adding a clean-up phase on
ensure which will schedule possible staled +deleted path deletion.
Note that we don't check the current state (if there is or not a
repo) in order to schedule the deletion. That's intentional
in order to leverage Gitlab::GitalyClient::NamespaceService#remove
idempotency and ensure consistency.