Remove a check for ICEControlling when parsing first TCP STUN message

[jeremija]

Your environment.

• Version: v2.0.0-rc.6
• Browser: Firefox

What did you do?

The check for ICE controlling is most likely not necessary. I couldn't establish a TCP Peer Connection that was initiated by the server, with passive server candidate and an active browser candidate.

What did you expect?

The connection should succeed.

What happened?

The connection failed because of this check:

ice/tcp_mux.go

Lines 147 to 156 in e8ac363

	// Firefox will send ICEControlling for its Active canddiate. We
	// currently support passive local TCP candidates only.
	//
	// TODO: not sure what will be sent for caniddate with tcptype S-O.
	_, err = msg.Get(stun.AttrICEControlling)
	if err != nil {
	m.closeAndLogError(conn)
	m.params.Logger.Warnf("No ICEControlling attribute in STUN message from %s to %s\n", conn.RemoteAddr(), conn.LocalAddr())
	return
	}

Will follow up with a PR shortly.