fix and rerun discussion_topics.group_category population

test plan:
  * run migrations

Change-Id: Idfb81d3c8524002a7222245896c196651dac07a6
Reviewed-on: https://gerrit.instructure.com/35405
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>