MM-12505: remove all things WebRTC (#9700)

model/websocket_message.go

@@ -37,7 +37,6 @@ const (
 	WEBSOCKET_EVENT_EPHEMERAL_MESSAGE       = "ephemeral_message"
 	WEBSOCKET_EVENT_STATUS_CHANGE           = "status_change"
 	WEBSOCKET_EVENT_HELLO                   = "hello"
-	WEBSOCKET_EVENT_WEBRTC                  = "webrtc"
 	WEBSOCKET_AUTHENTICATION_CHALLENGE      = "authentication_challenge"
 	WEBSOCKET_EVENT_REACTION_ADDED          = "reaction_added"
 	WEBSOCKET_EVENT_REACTION_REMOVED        = "reaction_removed"