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: I5e3b45f38d075fed1554bfe21606d36e8399db24
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/232249
Reviewed-by: James Williams <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
Reviewed-by: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
Tested-by: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
QA-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
Product-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>

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