remove icons from gradebook navigation pills

fixes CNVS-10627

test plan:
  * in a course with outcome gradebook enabled, navigate to
    gradebook and verify that the navigation pills only
    display text and not any icons.

Change-Id: Ic155deae1e5a8a62283786e117f64e61105c7247
Reviewed-on: https://gerrit.instructure.com/28873
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <banderson@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>