don't redirect deleted enrollment to crosslisted course

test plan:
 - create two courses, A and B
 - crosslist the default section of course A to course B
 - enroll a user into course B
 - as the user, go to course A ("/courses/{course A's id}")
  - you should be redirected to course B
 - delete the user's enrollment to course B
 - as the user, go to course A
  - you should not be redirected. (you may get a permission-denied
    error if the user doesn't have permission to read course A.)

fixes CNVS-16290

Change-Id: I1e154748039c7a2ba893f69530841b544dbb8d8a
Reviewed-on: https://gerrit.instructure.com/43088
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>