Remove `add_datasets_to_entity()` for Collection and Upload

AFTER the Harvard team addresses the corresponding changes in the portal-ui following the changes described in #754, we should remove the use of Collection.datasets and Upload.datasets from the index procedure.

search-api/src/hubmap_translator.py

Line 705 in a3a1013

def add_datasets_to_entity(self, entity):

There will be no need to compose a list of datasets:

search-api/src/hubmap_translator.py

Line 739 in a3a1013

entity['datasets'] = datasets

This issue becomes irrelevant due to #911