!con #3597 Remove hand over data message in cluster singleton

The initial version of the cluster singleton was running the singleton actor on the leader node, which could easily change, and therefore it made sense to be able to pass over state when switching singleton.
Then it was changed to run on the oldest member, and then this hand over feature doesn't add much value any more.
It is only used when leaving the oldest member gracefully, and the crash scenario must anyway be handled.
The implementation will be simplified by removing this feature.