allow switching grading from letter grade

fixes CNVS-12822

test plan:
  - use api to update assignment
    - change & remove grading type
    - change & remove gradind standard
    - change both & neither
  - go to assignment edit page
    - set a grading standard (not the default)
    - set grading type to letter grade & save
    - change grading type to points
      - ensure the change works
    - reset to letter grade and try again for other
      types

Change-Id: I24f521f9eeb1215d3d77d575371927667a88ee3e
Reviewed-on: https://gerrit.instructure.com/34226
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>