don't remove STI classes from ActiveRecord::Base.all_models
don't remove STI classes from ActiveRecord::Base.all_models

it prevents clearing schema caches. it was likely intended to
prevent truncating the same table multiple times in spec_helper,
but we haven't used all_models for that in a long time

Change-Id: Ife47ea6c565c1334c3049f91e3012b7e1f1de0f7
Reviewed-on: https://gerrit.instructure.com/73406
Reviewed-by: Brad Horrocks <7b5c5d1f2eba6c540b8e53f17d87a9bd7dbf604b@instructure.com>
Tested-by: Jenkins
Reviewed-by: Jacob Fugal <f862f167b85d41b225785c70d70808bc7337c1fe@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
QA-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

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