Remove the QThread::terminated() signal

Remove the QThread::terminated() signal

The signal is removed from the API; all references to it are removed
from documentation; the unit test that checks for its emission is
modified to listen for QThread::finished() instead.

The QThreadPrivate::terminated flag is also removed, as it served no
purpose other than to trigger the emission of QThread::terminated()

As discussed at http://lists.qt-project.org/pipermail/development/2012-October/007216.html
the signal is not guaranteed to be emitted after every termination,
rendering it useless.

Change-Id: I7b0c45d7889da0d33875545331606f2208ee56fc
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>