speedgrader: remove pointless/occasionally incorrect tooltip

fixes CNVS-12093

Test plan:
  Make a late submission, open it in the speedgrader. Hover over the
  submission date (in the sidebar). You shouldn't get lied to.

Change-Id: I7ebd8af6d4a3fa3dfaff6dd86a73d63a4fe3c72b
Reviewed-on: https://gerrit.instructure.com/33128
Reviewed-by: Liz Abinante <labinante@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>