First make sure that we don't use that variable anymore. We don't seem to use it in SaaS as shown here:

Account.where.has { service_preffix != nil }.count
=> 0

Partially done in this GH PR

At first I (Marta) thought that we needed this for the backend_id of a Service, for this:
https://github.com/3scale/porta/blob/ddd3f230fd271cfcc7b053cf3055a8e19fcf6a7f/app/lib/backend/model_extensions/service.rb#L11-L13
https://github.com/3scale/porta/blob/ddd3f230fd271cfcc7b053cf3055a8e19fcf6a7f/app/models/service.rb#L258-L260
https://github.com/3scale/porta/blob/ddd3f230fd271cfcc7b053cf3055a8e19fcf6a7f/app/models/service.rb#L201-L206
Then that would mean that we currently can't delete a service in backend when its tenant is deleted because at that moment, we don't have this data anymore. But if we are not using the service_preffix as it seems from the SaaS DB, then the backend_id of the service by default is just its id and it works perfectly, but then we need to remove that column (service_preffix) from the code and from the DB for real.