spec: don't run content migration specs twice

also speed up large_courses specs (3:26 -> 0:26)

Change-Id: Ie2447b14d1829851762d1cd133adaaaeafe97a04
Reviewed-on: https://gerrit.instructure.com/44316
Reviewed-by: Hannah Bottalla <hannah@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jon Jensen <jon@instructure.com>
QA-Review: Jon Jensen <jon@instructure.com>