DA - enrollment deletion works

fixes CNVS-15750

test plan:
  - DA on
    - make a module and visit the modules page as a student
    - as the teacher unenroll the student from a section
    - there should be no error when you unenroll the student
  - DA off
    - same thing

Change-Id: I9c2356cdb034b25e63b9693a610849072513e49b
Reviewed-on: https://gerrit.instructure.com/41744
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>