delete outdated json string ids hack

Test plan:
  try changing the group on an assignment, make sure the proper group is
  still selected when you go back to edit

Change-Id: I6dd69b6728ef3059b0cfb9a59ed96a261901cc1b
Reviewed-on: https://gerrit.instructure.com/32718
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>