ignore current/final points column in gb upload

fixes CNVS-10591

test plan:
- disable weighting by assignment group
- download the gradebook.csv
- you should have a Current Points and Final Points columns
- upload the csv
- it should say no changes, not offer to create assignments for those two
  columns

Change-Id: I4d4cc28b5b9de8ee6579c63dc11f791309d295e2
Reviewed-on: https://gerrit.instructure.com/28822
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Reviewed-by: Cameron Matheson <cameron@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>