Remove SIS attributes from manually recreated enrollments

This also improves consistency with SIS batch imports, as prior SIS
enrollments recreated manually behave the same as first time manual
enrollments.

Fixes CNVS-12983

Test plan:
- Add a user to a course via SIS enrollment
- Remove said user with another SIS enrollment
- Add the user to the same course, but manually
- The enrollment should no longer have SIS attributes,
allowing it to be removed normally through the UI.

Change-Id: I13a56d27fa33c47dc147664010789665daab658e
Reviewed-on: https://gerrit.instructure.com/40540
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>