remove rails2 from active_polymorph

Change-Id: I3a7ca146f037d0c7fa46efc1dcb11ba483938dac
Reviewed-on: https://gerrit.instructure.com/39669
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Anthus Williams <awilliams@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>