allow deleting grade for pass/fail assignments

fixes CNVS-14805

test plan:
- create a pass/fail assignment, with >0 points possible
- go to the gradebook
- update a student to have a checkmark for the assignment (pass)
- keep clicking until you get back to a non-graded state
- it shouldn't show an error banner

Change-Id: Ia19190d55083056a28583d87f6e41ed42f976347
Reviewed-on: https://gerrit.instructure.com/39961
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Reviewed-by: Liz Abinante <labinante@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>