Remove unused(?) PdbxLoaderWorker class

[piehld]

The class PdbxLoaderWorker doesn't appear to be used anywhere in the RCSB codebase, whereas the partly redundant but more extensive class PdbxLoader is used.

It seems that the PdbxLoader class was created as a clone of PdbxLoaderWorker, and then further developed and used instead of the PdbxLoaderWorker class altogether. If this is really the case (testing and/or research needed), it would be good to remove the unused class from the PdbxLoader.py code, since it contains a lot of redundant code, wastes space, and leads to confusion in reviewing the code: https://github.com/rcsb/py-rcsb_db/blob/master/rcsb/db/mongo/PdbxLoader.py

[piehld]

This is obviously a low priority, but should be relatively easy to resolve during the next merge/release, so just wanted to make a note of it here so we don't forget.