Remove disconnectClientSignals() from ~QmlProfilerClientManager()

disconnectClientSignals only disconnects signals of objects that are
deleted directly afterwards. As deleting a QObject automatically
disconnects its signals it's unnecessary to explicitly do that before.

However, if QmlProfilerClientManager is deleted from the destructor of
QmlProfilerTool, receivers of those signals may have already been
deleted. This can lead to crashes.

Task-number: QTCREATORBUG-10634
Change-Id: I1e5ce7a6ac4b331fa423987e3a8faa3584b991d2
Reviewed-by: Kai Koehne <kai.koehne@digia.com>