Change to event-level beforeRedaction event for efficiency

To avoid an O(n^2) situation with every relations container trying to process
every redaction that may occur in a room, this switches to an event-level
notification, so that the specific relations container who cares can listen to
just the events it wants to know about.

src/models/room.js

@@ -1011,7 +1011,6 @@ Room.prototype._addLiveEvent = function(event, duplicateStrategy) {
         // if we know about this event, redact its contents now.
         const redactedEvent = this.getUnfilteredTimelineSet().findEventById(redactId);
         if (redactedEvent) {
-            this.emit("Room.beforeRedaction", redactedEvent, event, this);
             redactedEvent.makeRedacted(event);
             this.emit("Room.redaction", event, this);