don't show rubric assess scores in api on muted assignments

fixes CNVS-12403

test plan:
- create an assignment with a rubric, and mute it
- grade the assignment with the rubric for a student
- as the student, query the api for the submission object
- it should not contain the rubric assessment data

Change-Id: Id83a9444f06c59938cd1bd5cac7a6ddb4019d45f
Reviewed-on: https://gerrit.instructure.com/33316
Reviewed-by: Liz Abinante <labinante@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Reviewed-by: Cameron Matheson <cameron@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>