WCP-314 only send cancelation email if user actually removed

Condition sending the cancelation notice email on removeSession[Non]Chairs actually having had an effect.  This prevents sending unnecessary cancelation emails where the controller calls removeSession[Non]Chairs blindly without knowing whether the given user is a chair or non-chair (the current, and it seems perfectly reasonable, behavior of `ManageParticipantsController`).

This cherry-picks the change in https://github.com/Jasig/BlackboardVCPortlet/pull/3 into `uw-master` for potential benefit to Wisc via a local release.