reduce complexity of Course#infer_root_account
reduce complexity of Course#infer_root_account

somewhere along the line Rails fixed assignments to foreign keys
invalidating cached associations, so we no longer need to do
dances to ensure we're looking at the right object without
force reloading

Change-Id: Ia4576e377848a7d92f24ad6046e2ad26662fb6d5
Reviewed-on: https://gerrit.instructure.com/115653
Tested-by: Jenkins
Reviewed-by: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
QA-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

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