Remove empty abstract skeletal implementer classes

[khatchad]

Remove any abstract skeletal implementer classes that are empty as a result of the refactoring.

Related to #25, which deals with concrete or effectively-abstract skeletal implementers. Also related to #20, as there may be javadoc for the class. But, if the class is under version control, this may be easy to deal with.

[khatchad]

Actually, we can keep the compilation unit around to make it easier for users to manually merge javadoc.

[khatchad]

Progress in task_166-Remove_empty_abstract branch.