QDBusArgument: remove useless op<< overloads

... and remove misleading comments (these are overloads, not specializations).

The QList overloads do nothing different from the generic container
overloads. Remove them.

Only leave the QVariantList overload, because that converts to
QDBusVariant before serializing. Which means that this should
probably be templated on the container type, otherwise you get
different behavior for QList<QVariant> and, say, QVector<QVariant>,
which is surely wrong.

Change-Id: I215ba9891235b51304c2ed4041d3dbd003d69581
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>