prefer AR magic over Shard#partition_by_shard

in User#update_account_associations, instead of
partitioning ids ourselves, harness the magical
cosmic powers of find_all_by_id

Test plan: specs

Change-Id: I5392d6ae6216ac3e940f5ce063ce5d7575ad6f3d
Reviewed-on: https://gerrit.instructure.com/28532
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Anthus Williams <awilliams@instructure.com>
QA-Review: Anthus Williams <awilliams@instructure.com>