remove duplicate callbacks in the Rubric model
remove duplicate callbacks in the Rubric model

test plan:
- basic rubric regression test

Change-Id: I2f0f7f1f34ccf15bc9673fca77e9b5625ddc445d
Reviewed-on: https://gerrit.instructure.com/27873
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Mike Nomitch <74443f6f110b6b7b934c89126258ed3d843d673f@instructure.com>
Reviewed-by: Cameron Sutter <853bcc0cb8f5834b7805e8eb9a762054be43c731@instructure.com>
QA-Review: Amber Taniuchi <ef4bfd1e6d278f6bc7d4a478f02be2cb9581a0a5@instructure.com>
Product-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>

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