There's a VoiceUser cleanup procedure bound to the User's cleanup
routine in Meteor's server-side. That cleanup is _silent_ and does not
use a dedicated modifier from voice-user et al, which is not
straightforward and might waste a few minutes of understanding what's
happening when debugging audio collections.
This commit centralizes that cleanup in a new clearVoiceUser modifier in
voice-user as well as logs when it works.
