Remove "update" parameter in favour of "sid"

The "update" parameter was used to know if an offer should be treated by
the clients as a renegotiation or a new connection. However, since the
introduction of "sid" this is implicitly known (if the "sid" matches an
existing connection then it is an update, otherwise it is a new
connection), so that specific parameter is no longer needed.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>

api_signaling.go

@@ -655,7 +655,6 @@ type AnswerOfferMessage struct {
 	RoomType string                 `json:"roomType"`
 	Payload  map[string]interface{} `json:"payload"`
 	Sid      string                 `json:"sid,omitempty"`
-	Update   bool                   `json:"update,omitempty"`
 }
 
 // Type "transient"