Bug 1041124 - [Messages][Refactoring] Make use of EventDispatcher in MessageManager. r=julien

Well at one point, deleteMessage could only take one id, and we did the dirty work here. Now that Gecko knows how to handle several ids, we only wired deleteMessages to deleteMessage. If we don't use it anywhere else, we can remove it.

Might make sense to move the comment that still applies to deleteMessage though. And/or rename existing deleteMessage to deleteMessages?