Don't store transceiver MID

These are generated at runtime (depending if we are doing Plan-B or
Unified Plan). Currently this field is actually unused, so just getting
rid of it.

Relates to #106

peerconnection.go

@@ -23,10 +23,6 @@ import (
 	"github.com/pion/webrtc/v2/pkg/rtcerr"
 )
 
-const (
-	newTransceiverMid = "NEW"
-)
-
 // PeerConnection represents a WebRTC connection that establishes a
 // peer-to-peer communications with another PeerConnection instance in a
 // browser, or to another endpoint implementing the required protocols.
@@ -579,7 +575,6 @@ func satisfyTypeAndDirection(remoteKind RTPCodecType, remoteDirection RTPTransce
 	}
 
 	return &RTPTransceiver{
-		Mid:       newTransceiverMid,
 		kind:      remoteKind,
 		Direction: RTPTransceiverDirectionInactive,
 	}, localTransceivers
@@ -642,7 +637,7 @@ func (pc *PeerConnection) addAnswerMediaTransceivers(d *sdp.SessionDescription)
 			for {
 				// keep going until we can't get any more
 				t, localTransceivers = satisfyTypeAndDirection(kind, direction, localTransceivers)
-				if t.Mid == newTransceiverMid {
+				if t.Direction == RTPTransceiverDirectionInactive {
 					break
 				}
 				mediaTransceivers = append(mediaTransceivers, t)
@@ -1338,7 +1333,6 @@ func (pc *PeerConnection) AddTrack(track *Track) (*RTPSender, error) {
 		)
 	}
 
-	transceiver.Mid = track.Kind().String() // TODO: Mid generation
 	return transceiver.Sender, nil
 }
 

peerconnection_go_test.go

@@ -374,7 +374,7 @@ func TestPeerConnection_AnswerWithoutOffer(t *testing.T) {
 
 func TestPeerConnection_satisfyTypeAndDirection(t *testing.T) {
 	createTransceiver := func(kind RTPCodecType, direction RTPTransceiverDirection) *RTPTransceiver {
-		return &RTPTransceiver{Mid: newTransceiverMid, kind: kind, Direction: direction}
+		return &RTPTransceiver{kind: kind, Direction: direction}
 	}
 
 	for _, test := range []struct {

rtptransceiver.go

@@ -8,7 +8,6 @@ import (
 
 // RTPTransceiver represents a combination of an RTPSender and an RTPReceiver that share a common mid.
 type RTPTransceiver struct {
-	Mid       string
 	Sender    *RTPSender
 	Receiver  *RTPReceiver
 	Direction RTPTransceiverDirection