If a participant is removed from an event, all confirmations should be removed

[mconf-daileon]

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Author Name: Leonardo Daronco (Leonardo Daronco) Original Redmine Issue: 1524, http://dev.mconf.org/redmine/issues/1524

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If a participant is removed and there is a pending confirmation that wasn't sent yet, this confirmation will fail because the participant doesn't exist anymore.

To fix this and make the database more consistent, when a participant is removed, all the associated ParticipantConfirmation's should also be removed.