don't show deleted group categories in the api

fixes CNVS-15450

test plan
- create a group category
- delete the group category
- ensure that the group category cannot be shown via the api
 (GET /api/v1/group_categories/:id)
- ensure that a group cannot be added to the group category via
 the api (POST /api/v1/group_categories/:id/groups)

Change-Id: Ieb0cfdefafb64f46322079b17a50d6a837d1630d
Reviewed-on: https://gerrit.instructure.com/45597
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Andrew Butterfield <abutterfield@instructure.com>
QA-Review: Derek Hansen <dhansen@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>