remove Assignment.assignment_scope_for_draft_state

closes CNVS-17254

Test plan:
  * the gradebook should not show unpublished assignments
  * grades from the enrollment csv should be correct (they should not
    consider unpublished assignments)

Change-Id: I5a5405ff33146beeeba5453eff388d2bf806b427
Reviewed-on: https://gerrit.instructure.com/45250
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>