remove new_teacher_registration notification code

This notification is no longer sent, but we kept this code around for a
bit to allow already-queued messages to succeed.

closes CNVS-15219

Change-Id: I67f50df8f15c0e4d02f4fc262c0dedad831df96b
Reviewed-on: https://gerrit.instructure.com/40383
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>