drop notification_policies.broadcast

it's never used

NotificationPolicy#preference also becomes obsolete, because NOT NULL
and FK ensure that #communication_channel can never be nil; it's
also never used

Change-Id: I0669d3b89964ee6f6d4c6a62e348e276bb7ef315
Reviewed-on: https://gerrit.instructure.com/39446
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>