Consider removal of Choice types from the data model

Choice types were an early modeling strategy that reflected cases where there was one reference to a set of possible entities. A exemplar case is Assessment's reference to either a Program or a Section:

http://schema.ed-fi.org/datahandbook-v31/Ed-Fi-UDM-Handbook-Index.html#/SectionOrProgramChoice2504

In practice, the API implementations have not enforced the either/or and modeled this simply as a collection of optional attributes for each choice (see the Ed-Fi ODS/API as an example).

Further, the Assessment Outcomes APIs specification simply dropped the element: https://techdocs.ed-fi.org/display/EFDSRFC/ED-FI+RFC+15+-+ASSESSMENT+OUTCOMES+API

This change would break the XSD/XML bindings where the concept of "choice" is a native pattern and where that patterns is used. However, in the Ed-Fi ecosystem based on API exchange the abstraction has not been used and seems to have little value.