drop unused enrollment index

fixes CNVS-11521

test plan
 - migrations should run

Change-Id: I04cd9ce236fa83afd9c5eacb2abdab39eeb3d3ed
Reviewed-on: https://gerrit.instructure.com/31099
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>