QEventDispatcherQPA: processEvents should only be called once

This caused a "deadlock" in tst_qobject::moveToThread

The problem was that QEventLoop::quit was called from the first
prcessed event in that loop, put calling process event a second time
deadlock because there is no more event to process

(In practice, some event can come from the window manager or the
inputs, but they may as well not come)

Change-Id: Ia469110eb9c9de57669e80cf19e933f410e469a4
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>

src/platformsupport/eventdispatchers/qeventdispatcher_qpa.cpp

@@ -70,7 +70,6 @@ bool QEventDispatcherQPA::processEvents(QEventLoop::ProcessEventsFlags flags)
     bool didSendEvents = QWindowSystemInterface::sendWindowSystemEvents(this, flags);
 
     if (QEventDispatcherUNIX::processEvents(flags)) {
-        QEventDispatcherUNIX::processEvents(flags);
         return true;
     }