Deleted domains should not be considered as tenants

Steps to reproduce:

1. Install Multi-tenant.
2. Create a new domain.
3. Delete it.
4. Activate Tenant isolation.
5. Create a new user.
   -> In the Tenant ID drop down list, the deleted domain is displayed.

Another slightly different but similar bug happens when you activate multi-tenant, create a domain, then delete it: it's still visible as a tenant.