DA - remove deleted assignments from sql view

fixes CNVS-15511

test plan:
  - turn DA off and make an assignment then delete the assignment
  - note the assignment's id
  - open the rails console and run this
    - "AssignmentStudentVisibility.where(assignment_id: XXXXX)"
    - instead of XXXXX, you should write the assignment's id
    - the result of that command should be an empry array => []

Change-Id: I2f09560ce46bd4682e9bf671b4c116508a6f4b49
Reviewed-on: https://gerrit.instructure.com/41114
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>