remove submission comment mark as read notification preference

fixes CNVS-14178

test plan
- view notification preference page
- ensure that checkbox for submission comment 'mark as read'
 setting is not visible
- ensure that submission comment tooltip does not mention the
 checkbox

Change-Id: Ib859aea0fcf5a4e1672e02665725ffa31d99349e
Reviewed-on: https://gerrit.instructure.com/37579
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Tested-by: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>