I have noticed in https://github.com/twbs/bootstrap/commit/e342142b61aacc187c776052bde93e414eb25db7 some remaining $accordion-* SCSS variables (whose some associated with a // todo).
I'm not sure if it was on purpose so this PR proposes to finish this transformation. Don't hesitate to tell me if some of them should stay as it is and not be changed.
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/cc @mdo