kill assignment global nav

fixes CNVS-13317

test plan:
 > assignments global navigation should be gone
 - type in '/assignments' as the URL manually
 > it should redirect you to the user dashboard

Change-Id: I6f26b825119268b62164ee37897be86909682a17
Reviewed-on: https://gerrit.instructure.com/39788
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>