GDBusConnection: Don't store GCancellable in qdata

Now that we're using g_simple_async_result_set_check_cancellable() we
no longer need this terrible hack of carrying the GCancellable on the
GSimpleAsyncResult using qdata. See bug 672013 for more details.

https://bugzilla.gnome.org/show_bug.cgi?id=672013



Signed-off-by: David Zeuthen <davidz@redhat.com>