modify update to remove in_batches
modify update to remove in_batches

in_batches is terrible, it caused a huge select that we didn't need.
Let's slice on assignments instead.

test plan:
 - specs pass

Change-Id: Ie238b1a0e8490a139c2cd8239fb60a63e2b0e579
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/232053
Reviewed-by: Spencer Olson <7cc0890d6f1f70810a023e7173eca5724d6c8e49@instructure.com>
Reviewed-by: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
QA-Review: Keith Garner <a1a80143a840c140f31c3495ebbd9ec906e09fed@instructure.com>
Product-Review: Keith Garner <a1a80143a840c140f31c3495ebbd9ec906e09fed@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms