Dropped not existing keyword attribute `enabled`.

Because enabled_only is the default for `OGDSService.all_org_units` to passing a keyword argument is needless (See https://github.com/4teamwork/opengever.ogds.models/blob/rework-mandanten-concept/opengever/ogds/models/service.py#L58).
Merge pull request #398 from 4teamwork/pg_fix_user_sort_dict

Dropped not existing keyword attribute `enabled`.