Cause assignments within modules to be reported as links by screen readers

It's not necessary for the items within modules to be headings as well, so
this behavior has been removed.

Fixes CNVS-11493

Test Plan:
* Visit a courses module page while logged in as an instructor
* If necessary, create a module and add some items to it.
* Using a screen reader, verify that the module name is read as a heading.
* Verify that the names of items within each module are read as links.
* Verify that all other clickable elements on the page are reported by screen
  readers as being either links or form elements, or offer an alternative
  means of being accessed.

Change-Id: If213ecc14144114031454b5cd902de4fd23d33c5
Reviewed-on: https://gerrit.instructure.com/31080
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Reviewed-by: Sterling Cobb <sterling@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>