hide rubric icons in speedgrader

When viewing a rubric in speedgrader, the "+", "X" and pencil icons
will no longer be visible.
These same icons were also showing up and non-functional at the
courses/:course_id/rubrics/:rubric_id page BEFORE clicking
the 'Edit Rubric' button. Those should not be visible anymore.

closes CNVS-16334

test plan:
- Have a course with at least 1 student
- Create an assignment that has a rubric attached
  (does not matter if the rubric is being used for grading or not)
- Have a student submit to the assignment
- In Speedgrader, view the rubric and verify the "+", "X"
  and pencil icons are no longer present when hovering over the table.
- View the individual page for the rubric (for example,
  /courses/1/rubrics/1)
- Verify the "+", "X" and pencil icons are no longer present
  when hovering over the table, before clicking the 'Edit Rubric'
  button.
- Click the 'Edit Rubric' button. Verify the icons show up when
  hovering over the table.

Change-Id: Ifbc8a57bd133fe5fe2bd3f3ee07b9b350d6fb487
Reviewed-on: https://gerrit.instructure.com/43104
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Spencer Olson <solson@instructure.com>