Remove jsonschema2pojo-scalagen

[joelittlejohn]

This is an experimental feature using an old and outdated Java to Scala converter. It doesn't create useful Scala, and it has serious flaws (#936, #1172).

A JSON to Scala plugin is really a whole new library, and our quick hack of converting generated Java to Scala produces terrible results. Any actually useful attempt at Scala generation would produce case classes, use Option for things that are not required, and do a variety of other things.

To avoid propagating this terrible Scala I'm removing this feature until and unless we produce something useful.