remove dead overrides code

closes CNVS-10536

test plan:
- basic regression test of create/viewing multiple
  due dates for students and teachers

Change-Id: I3c5e4707efd2b0e27908efb4f4730b2778dd2454
Reviewed-on: https://gerrit.instructure.com/28648
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Matheson <cameron@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>