don't fetch assignments for assignment group discussion option
don't fetch assignments for assignment group discussion option

test plan:
* the drop-down for assignment groups when creating
 a graded discussion should appear normally

closes #CNVS-38774

Change-Id: I68dda521ea2695b5e0f4f7526eade15ccebe82eb
Reviewed-on: https://gerrit.instructure.com/123884
Tested-by: Jenkins
Reviewed-by: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
QA-Review: Deepeeca Soundarrajan <5bcd4b623f00b7fc2942cd1c65c245b30a48a61f@instructure.com>
Product-Review: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>

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