remove unused actions in rubric_assessments_controller

the index and show actions are unused. the index view was
removed awhile ago in 91ce600f.

Change-Id: Ib84fc2e9e524aec3143a4395d15f9c1320798648
Reviewed-on: https://gerrit.instructure.com/29983
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Jon Willesen <jonw@instructure.com>
QA-Review: Jon Willesen <jonw@instructure.com>