This PR removes dummy serialisers for `StateSetUpdater`, `NodeCallback` and the respective `META` macros that trigger serialisation requirement here.
`StateSetUpdater` and `NodeCallback` are just base classes that can not be used on their own, so there is no need to incorporate them into serialisation. These changes might have minor effects on derived classes that forget to override `className()`, `libraryName()` through `META`, but it makes hardly a difference to now serialise such classes as a dysfunctional `osg::Callback` instead of a dysfunctional `SceneUtil::NodeCallback`.
With this PR we refactor `SceneUtil::KeyframeController` not to require `virtual osg::Callback` inheritance. I suppose such `virtual` overhead is not justified here because it negatively impacts many other classes we derive from `osg::Callback`.