remove discussion replies cruft

fixes CNVS-11990

test plan
- ensure nothing changed, especially things having to do with
 inboxes and discussions

Change-Id: I07cfbf17ca04459f5048bf6fe8befc200dbfd965
Reviewed-on: https://gerrit.instructure.com/32441
Reviewed-by: Braden Anderson <banderson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>