remove shutdown method on the Connection (#4249)

There's no need to have a dedicated shutdown method, as the use case
(shutting down an outgoing connection attempt on context cancellation)
can be achieved by using Connection.destroy.

server.go

@@ -24,7 +24,6 @@ var ErrServerClosed = errors.New("quic: server closed")
 // packetHandler handles packets
 type packetHandler interface {
 	handlePacket(receivedPacket)
-	shutdown()
 	destroy(error)
 	getPerspective() protocol.Perspective
 }
@@ -45,7 +44,6 @@ type quicConn interface {
 	getPerspective() protocol.Perspective
 	run() error
 	destroy(error)
-	shutdown()
 }
 
 type zeroRTTQueue struct {