revert DA grading to fix grade calc performance regression

this reverts part of b4840c75cc2902159fc7fa826facc43921d2ac2b

fixes CNVS-14840

test plan:
- with differentiated assignments off, grading should work correctly
- in a course with many assignments, you should be able to download the
  gradebook csv
- with differentiated assignments on, grading will not work correctly, this is
  intentional.

Change-Id: Id910ff28763ba13c529a21d4c125a164e094aeac
Reviewed-on: https://gerrit.instructure.com/39436
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>