eliminate unnecessary discussion_topic queries in assignment_json

refs CNVS-16051

Test plan:
  the assignments index page should still work for discussion topic
  assignments (the gear menu, etc.)

Change-Id: Ia94755eb5189a6c2b7f26fd115d3461c0ff31d4d
Reviewed-on: https://gerrit.instructure.com/42455
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>