Cleanup - Data Group

It looks like `DataGroup` is not used in the project but there are some references to it. We can remove it as part of the cleanup.

You can see that on django admin DataGroup is empty: https://www.codabench.org/admin/datasets/datagroup/

DataGroup used in the code: https://github.com/search?q=repo%3Acodalab%2Fcodabench%20DataGroup&type=code

