drop unused column from enrollment terms

Change-Id: Ib6cb6d6e9244b6c3be9602fc3fa2c33ea866e41d
Reviewed-on: https://gerrit.instructure.com/42395
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>