spec: remove assignment tab spec

nuke the assignment header tab spec since we removed that tab
from canvas

Change-Id: I13e60548e2dd32648233b85c877309cc7f46389d
Reviewed-on: https://gerrit.instructure.com/44223
Product-Review: Kyle Rosenbaum <kyler@instructure.com>
Reviewed-by: Kyle Rosenbaum <kyler@instructure.com>
QA-Review: Kyle Rosenbaum <kyler@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>