don't use write_attribute for prior_enrollment

refs CNVS-15830

test plan:
 * view the list of prior enrollments for a course
 * it should work

Change-Id: Ifb92a7a5c8f2b2a8382c2d8666038f9c4e7be4a3
Reviewed-on: https://gerrit.instructure.com/41977
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>