Fix inheritance mapping of Person, Student and Instructor in Contoso uni app

The mapping should be a single Person object with a discriminator that allows a Student or an Instructor to be stored in CB.

The current modeling adds three docs for each student, person and instructor. Person is empty, so this returns nothing:

SELECT `c`.`ID`, `c`.`FirstName`, `c`.`LastName`, `c0`.`HireDate`

FROM `contoso`.`person`.`person` AS `c`

INNER JOIN `contoso`.`person`.`instructor` AS `c0` ON `c`.`ID` = `c0`.`ID`

ORDER BY `c`.`LastName`