Set role/group only to existing labels

[tabergma]

If you are using entity roles/groups, the model can predict something like

    "predicted_entities": [
      {
        "entity": "city",
        "start": 43,
        "end": 51,
        "role": "O",
        "value": "new york",
        "extractor": "DIETClassifier"
      }

The role is set to O, but there is no O role label in the training data. It basically refers to "no role label detected". Instead of setting the role to O, we should remove it from the entity returned by the model.