remove the new teacher registration email

This email only went to site admins, and isn't useful.

This commit stops new notifications being created, a subsequent commit
will remove the no longer used code and data.

refs CNVS-15219

Change-Id: Ibd8c88c7310cf3dcfb06cf0c782f2b3cf571d843
Reviewed-on: https://gerrit.instructure.com/40375
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>