Use Plexus API instead of the outdated json file

@eighthave added the Plexus json file in modules/link_to_services.py in this commit, which is now outdated. Plexus has transitioned to an API. The json file is also marked for deletion. I had also emailed him about the same a few weeks ago along with all the GET routes & other info required.

Once this is completed, please let me know so we can delete the json file from the Plexus repo.