don't remigrate migrated preference value
don't remigrate migrated preference value

Change-Id: If0f5869bc7dc33006e8445253ba73793cb19423e
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/231980
Product-Review: Gentry Beckmann <f487939f789774cca922ca5b886632de73477ea0@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
QA-Review: James Williams <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>

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