Simplify model printing modes (#7049)

Simplify model printing modes (#7049)

This removes the model printing mode options::ModelUninterpPrintMode::DtEnum which was previously used to print uninterpreted sorts as enumeration datatypes in the model. This mode was to my knowledge never used and moreover will not be easy to implement in the API.

This is work towards finalizing the API for models.

This PR also removes a file that I failed to delete in a recent PR.