CLC-3731 CLC-3668 CLC-3510 Fix duplicated secondary sections

https://jira.ets.berkeley.edu/jira/browse/CLC-3731
https://jira.ets.berkeley.edu/jira/browse/CLC-3510
https://jira.ets.berkeley.edu/jira/browse/CLC-3668

The nested-section implicit instructor assignments didn't account for existing *explicit* instructor assignments, resulting in duplicated section entries for My Academics and bCourses Course Provisioning.