gdbus: Remove root node 'name' attribute in introspection

gdbus: Remove root node 'name' attribute in introspection

generate_introspection_xml generates the root <node> tags with a
'name' attribute. This seems to be a valid attribute but it is not
consistent with the way the D-Bus daemon generates empty nodes.

For example if we register "/foo/bar", D-Bus daemon will generate for
"/foo" a introspection which looks like this:

<!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
"http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
<node>
  <node name="bar"/>
</node>

and generate_introspection_xml generates for "/foo/bar":

<!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
"http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
<node name="/foo/bar">
</node>

Just don't add the 'name' attribute to the root node.  The GLib
binding for D-Bus does it the same way.