remove unneeded check for destination connection ID in the client

The packetHandlerMap routes packets to the client based on the
destination connection ID, so we can be sure that packets that are
handled in the client have the right connection ID.

client.go

@@ -308,11 +308,6 @@ func (c *client) handlePacketImpl(p *receivedPacket) error {
 		return err
 	}
 
-	// reject packets with the wrong connection ID
-	if !p.hdr.DestConnectionID.Equal(c.srcConnID) {
-		return fmt.Errorf("received a packet with an unexpected connection ID (%s, expected %s)", p.hdr.DestConnectionID, c.srcConnID)
-	}
-
 	if p.hdr.Type == protocol.PacketTypeRetry {
 		c.handleRetryPacket(p.hdr)
 		return nil