allows deletion of last section due date

fixes CNVS-13183

test plan
- in rails 2
- in a course with multiple sections that have at least 1 student enrolled
- once with an assignment, and once with a graded discussion topic
- create an assignment with a due date for 2+ sections and 'Everyone else'
- delete the section due dates and save the assignment
- verify that the due dates were actually deleted
- repeat for rails 3

Change-Id: I2c57af7cdf08e39f93d682193a2c4ed5aef82ba3
Reviewed-on: https://gerrit.instructure.com/35295
Reviewed-by: Cameron Matheson <cameron@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Liz Abinante <labinante@instructure.com>