don't order enrollments associations

it was a useless order anyway, and in rails 4 the order isn't removed
in has_many :through, :uniq, causing sql errors

Change-Id: Ieda66f3ab5a5c75c15fafac2eeb153e305de39e0
Reviewed-on: https://gerrit.instructure.com/41714
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>