Help: Make "System Information" a QDialog again

Making it a QWindow meant that Alt-Tab will lead to the main window and
right back to "System Information" because this is modal. This
incovenience outweighs the convenience of having a "Maximize" button.

This reverts a part of 40d7399755

Task-number: QTCREATORBUG-20513
Change-Id: Idcea54ee2f60f9f7efde7d25ce0c305b87f445dd
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/help/helpplugin.cpp

@@ -758,7 +758,6 @@ void HelpPluginPrivate::slotSystemInformation()
 {
     auto dialog = new DialogClosingOnEscape(ICore::dialogParent());
     dialog->setAttribute(Qt::WA_DeleteOnClose);
-    dialog->setWindowFlags(Qt::Window);
     dialog->setModal(true);
     dialog->setWindowTitle(HelpPlugin::tr("System Information"));
     auto layout = new QVBoxLayout;