spec: workaround mysql enforcing foreign keys within a single delete

refs CNVS-7032

Change-Id: I99756382e00335457ba9540ecabaafa85e69fbca
Reviewed-on: https://gerrit.instructure.com/28647
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>