Remove Classification worker version ID attributes from the API

Please remove the following fields from the API:

CreateClassification
- worker_version_id ForbiddenField in the request body
- worker_version_id in the response
CreateDatasetElement
- element.classifications[].worker_version in the response
ListDatasetElements
- element.classes[].worker_version in the response
ListElementChildren
- classes[].worker_version in the response
ListElementParents
- classes[].worker_version in the response
ListElements
- classes[].worker_version in the response
ListSelection
- classes[].worker_version in the response
RejectClassification
- worker_version_id in the response
RetrieveElement
- classifications[].worker_version in the response
ValidateClassification
- worker_version_id in the response