remove unused, deprecated method multiple_due_dates_apply_to

fixes CNVS-10361

test plan:
- tests pass
- you can still create an assignment with multiple due dates

Change-Id: Ied41b61cea32cf23d0ac86e3b4a3b3428003c6fd
Reviewed-on: https://gerrit.instructure.com/28330
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Liz Abinante <labinante@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>