don't use write_attribute for primary_enrollment data

refs CNVS-15830

test plan:
 * the courses menu should not asplode

Change-Id: Ibec8f2496df476767472bf2245edde63909e7b16
Reviewed-on: https://gerrit.instructure.com/41980
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>