Remove "id" from RetrievalSource object

[sierra-moxon]

It takes a long time to generate in translator-ingests, its generated inconsistently, and its not used.
Discussion (if any) below.

[RichardBruskiewich]

As suggested in Slack, should we simply introduce a new (super simple root) hierarchy upon which to root such Biolink Model classes as 'RetrievalSource' (and other related ones, like StudyResult and AffinityMeasurement)? Call the (abstract) parent something like "embedded entity" or "component" but only have the simplest of slots (not including the 'id' field)?