ignore irrelevant enrollments in gradebook2
ignore irrelevant enrollments in gradebook2

fixes CNVS-28685

Test plan:
  * set up a course where a student is an observer of another student
    (but also a student in the course himself)
  * the gradebook should load

Change-Id: I421ae51964eaecc2208740ffff99cfef2271eb0c
Reviewed-on: https://gerrit.instructure.com/76951
Tested-by: Jenkins
Reviewed-by: Keith T. Garner <a1a80143a840c140f31c3495ebbd9ec906e09fed@instructure.com>
Reviewed-by: Derek Bender <d24d8a7ee520e34b1cec38800225d9f36b552df7@instructure.com>
QA-Review: Amber Taniuchi <ef4bfd1e6d278f6bc7d4a478f02be2cb9581a0a5@instructure.com>
Product-Review: Keith T. Garner <a1a80143a840c140f31c3495ebbd9ec906e09fed@instructure.com>

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