Remove obsolete conversion work-around.

The conversion from QJSValue to QJsonArray seems to work just fine
with Qt 5.4. Originally, this work-around was introduced to fix the QML
tests, which still pass without this code now.

Change-Id: Id52a5a16ebe25914f01d597778152e0595c9f1f4
Reviewed-by: Lars Knoll <lars.knoll@digia.com>