only send cancellation email if user actually removed

Condition sending the cancellation 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`).

In practice this change de-duplicates cancellation emails: whereas currently the implementation will send an invitee two cancellation emails when he or she is removed from the invitees list (one for the attempt to remove from the set of chairs and one for the attempt to remove from the set of non-chairs), with this change the portlet will only send one such email (for whichever attempt had an actual effect).