revert broken activerecord scope in mail processor

test plan:
  * run specs

Change-Id: I16d63a4af72b1bce19ad52625191a922e84983c2
Reviewed-on: https://gerrit.instructure.com/28523
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Zach Pendleton <zachp@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>