slots are created when using `--use-attributes`

[djarecka]

When option --use-attributes attributes are added to the class, but the model still has all the attributes in the slots section:

slots:
  tissue sample label:
    from_schema: http://example.com/test
  amplified cDNA label:
    from_schema: http://example.com/test
  amplified cDNA amplified quantity ng:
    from_schema: http://example.com/test

classes:
  tissue sample:
    in_subset:
    - bican, library_generation
    from_schema: http://example.com/test
    is_a: material sample
    mixins:
    - prov entity
    attributes:
      tissue sample label:
        from_schema: http://example.com/test
        range: integer