remove :order on assignment stats queries

fixes CNVS-10564

test plan:
- basic regression test of the student course grades page.  basically it should
  load and the avg/min/max graphs should still work.

Change-Id: I3eb3b60b4b2f0165497d8ecc5512712133cc9803
Reviewed-on: https://gerrit.instructure.com/28720
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>