Retire unused code from the opendap.dap4 package

There is a broken/incomplete implementation of DAP4 types in `opendap/dap4`
Some of these classes are actually in use - but most of them are things that were part of the WCS-2 work that is being/has been retried.

Once we have said goodby to WCS these classes are just going to be confusing cruft.


Keep what we need, retire the rest.