fix delete button on new assignment details page

fixes CNVS-11824

test plan:
- go to the assignment index page
- click the 'new assignment' button
- on the new assignment details page, click the cog
- click 'delete'
- it should take you back to the index page, and not save the new assignment

Change-Id: Ifbbb151b40e526cd337ce809ea12d0df4371143c
Reviewed-on: https://gerrit.instructure.com/32034
Reviewed-by: Cameron Matheson <cameron@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Clare Strong <clare@instructure.com>