Removes redundant saving code

Removes code related to the previous method of saving. See previous commits
for context.

- Remove saving from DataSet, it is being done in NewDataSet
- Remove Record, it is no longer necesarry, dicts are used instead.
- Remove test code associated with the dead code.
Add delete_dataset method to mongo storage engine

I missed this one in earlier commits and only realised when I tried to
remove the Database class from the write API.