don't build_assignment on quizzes when importing

test plan:
* copy a course with an unpublished quiz (also try
while adjusting or removing dates)
* should not create "quiz-less" assignments that
 break the assignments page

fixes #CNVS-15477

Change-Id: Ib7d39735936956ea77cc555aeafdcacec2cccfcf
Reviewed-on: https://gerrit.instructure.com/41445
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>