remove some of the less useful associations from export

delayed_messages, exports, imports, migrations,
stream_items, etc.

Change-Id: I6eb63457ad496d7aaf56fc9ec8a9602a5679fdd7
Reviewed-on: https://gerrit.instructure.com/35215
Reviewed-by: Braden Anderson <banderson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Anthus Williams <awilliams@instructure.com>
QA-Review: Anthus Williams <awilliams@instructure.com>