QmlDebug: simplify the qml debug connection

We're using exactly one method of the QIODevice API in
QmlDebugConnection: errorString(), and that is just passing the error
string of the internal QTcpSocket.

It's hard to justify all the boilerplate code to implement QIODevice
and the hackery around close() for that.

Change-Id: I90b90c67c60faa73e288c1bbcd962ea64db8f76a
Reviewed-by: Kai Koehne <kai.koehne@digia.com>