Do not specialize the qobject_cast template for service interfaces.

This fixes issue #19.

The intent was to add support for qobject_cast<Interface*>(qobject)
calls for all micro service interfaces automatically if Qt is used.
However, this never fully worked because the QObject derived class
needs a Q_INTERFACES macro which requires moc to find an explicit
Q_DECLARE_INTERFACE macro for the used interface. By removing the
qobject_cast specializations, Q_DECLARE_INTERFACE and
US_DECLARE_SERVICE_INTERFACE can be used in parallel and existing
Qt classes can also be marked as a micro service interface (no
qobject_cast specialization clashes any more).
Merge branch '19-declare-service-interface-with-qt' of https://github.com/saschazelzer/CppMicroServices