remove quiz_submissions from json serialization of quizzes

fixes CNVS-11983

we can't load and serialize all quiz_submissions at once since some quizzes
will have tens of thousands of submissions. This will need to be changed in a
future commit to use a better strategy for getting submissions

Change-Id: Iab1a5ce9eb7a08424df3e6262a1fcf1f25efae0e
Reviewed-on: https://gerrit.instructure.com/32271
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>
Reviewed-by: Stanley Stuart <stanley@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>