remove index on assignments.integration_id

It turns out the integration_id is not unique, and in addition,
is not used for lookup by SIS or pro serv.

Fixes SIS-287

Change-Id: Ie3a2e35fad064896d69fb980ba8103b0861f04c2
Reviewed-on: https://gerrit.instructure.com/35339
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>