Code cleanup: remove DatasetfieldDefaultValue and DefaultValueSet

[bencomp]

Overview of the Feature Request I suggest to remove the classes DatasetfieldDefaultValue and DefaultValueSet and references to these classes, because they appear to be unused.

What kind of user is the feature intended for? Developer

What inspired the request? As I'm trying to add unit tests that test the very core data model, I came across these classes that are only referenced from DatasetFieldType and themselves. As far as I can tell without any documentation in the code, there is no interaction with these entities from the UI or the API. The idea to add unit tests came from @pdurbin – because without them, it's hard to refactor. In the end, Hacktoberfest inspired me to see if I could contribute a little to Dataverse.

What existing behavior do you want changed? If my analysis is correct, removing these would not change any behaviour.

Any related open or closed issues to this feature request? I suggested removing a few other classes in #9009. My first reference to code quality, including documentation, was probably #775.

@sekmiller is the author of these classes; @landreev may also be interested in cleaning up.