remove alignments when a question bank is deleted

fixes CNVS-7236

test plan:
- create an outcome
- create a question bank and align the outcome to it
- delete the question bank
- try to delete the outcome - it should work

Change-Id: I41e61355ba406eb6cb0c4078bf265406575e6d90
Reviewed-on: https://gerrit.instructure.com/34911
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>