Remove user_lists, learning_path_parents entirely from serializers

[mbertrand]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/10089

Description (What does it do?)

Removes the learning_path_parents and user_list_parents fields entirely from learning resource serializers.
This will cause lots of failures on the openapi test but they can be ignored.

How can this be tested?

• Log in as an admin, go to the search page, add some courses to your Favorites and learning paths, refresh the page. You should see courses in your paths/lists marked as such before and after the refresh.
• Go to http://open.odl.local:8062/learningpaths. You should still be able to see your learning paths, the list of resources in each, etc.
• Go to http://open.odl.local:8062/dashboard/my-lists. You should still be able to see your userlists, the list of resources in each, etc.