update user cache on enrollment deletion

fixes CNVS-12427

test plan
 - enroll a user in a course
 - delete the enrollment
 - as the user make a GET api call to 
 - <canvas_url>/api/v1/courses (can use ?as_user_id=ID)
 - should return unauthorized

Change-Id: I949640ad9ad8d9d702ac3f7b79dd19f35f4f98b5
Reviewed-on: https://gerrit.instructure.com/35658
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>