Remove redundant Scan model

[steven-esser]

Now that we use VirtualCodebase, we should remove the Scan model and its related tests here: https://github.com/nexB/deltacode/blob/a09514623fc51222230f356a6f911f3e70086cdc/src/deltacode/models.py#L33

[Pratikrocks]

@MaJuRG thanks for pointing it out made a PR for the same