don't delete memberships when group is deleted

refs CNVS-10184

there is no reason to do this but it makes it impossible to grade group
assignments after the group is deleted

Test plan:
  * make a group assignment
  * make some submissions
  * delete the group category
  * you should still be able to grade the submissions in speedgrader
  * nothing should change from the student's perspective (students
    should *not* see the group anywhere they currently don't see the
    group)

Change-Id: I2c421ea5d88fe77679ddffc60fc3ddb420d33a23
Reviewed-on: https://gerrit.instructure.com/39945
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>