Don't use a Rails model in a migration test
From https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2589#note_36815839
The ApplicationSetting
model was changed in CE, which broke an older migration test (spec/migrations/update_authorized_keys_file_spec.rb) in EE. The model itself doesn't seem to be necessary for this test setup, so we should just not reference it.