Consider removing connection type setting for QPushButton

https://github.com/enthought/traitsui/pull/1303 sets the QPushButton click slot connection type to QueuedConnection in order to address missing/weird repainting issue on Qt 5 and OSX when a QPushButton is pressed.

The behaviour has been observed on OSX 10.15.5 and PySide2 5.15 and PyQt 5.14.

The closest unresolved bug I can find on Qt bug tracker is this: https://bugreports.qt.io/browse/QTBUG-68067

Once the issue is resolved upstream, we should consider removing the type setting and revert back to using the default auto connection.

So far I don't think it hurts to setting the connection type to queued connection actually. The only cost is that test code will need to be more diligent about forcing GUI events to be processed, but that would be good practice for test code anyway. I think we prefer using the default if it is good enough.