remove some associations from the set of exportables

mostly has_many_through associations, where we are
already covered because we export the through
association as well

Change-Id: I95deb0f3c55059a480f1d77aa70ad32701267f91
Reviewed-on: https://gerrit.instructure.com/36335
Reviewed-by: Braden Anderson <braden@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Anthus Williams <awilliams@instructure.com>
QA-Review: Anthus Williams <awilliams@instructure.com>