Do not send DD extension if ID is 0. (#3339)

* disable temporal layer scaling

* remove dummy start

* do not add 0 id

* remove tests

* clean up

pkg/sfu/downtrack.go

@@ -874,7 +874,7 @@ func (d *DownTrack) WriteRTP(extPkt *buffer.ExtPacket, layer int32) error {
 	}
 
 	// add extensions
-	if tp.ddBytes != nil {
+	if d.dependencyDescriptorExtID != 0 && tp.ddBytes != nil {
 		hdr.SetExtension(uint8(d.dependencyDescriptorExtID), tp.ddBytes)
 	}
 	if d.playoutDelayExtID != 0 && d.playoutDelay != nil {