feat: add unregister function for subsystem

[vsoch]

An unregister function should delete a cluster from the in memory graph database. I'm curious if go garbage collecting will handle this as long as an object (the cluster root) is disconnected from any running context. I think I can test that with this tool: https://dave.cheney.net/2014/07/11/visualising-the-go-garbage-collector

E.g.,:

1. Run server with this visualizer
2. Register a new cluster
3. Unregister and see if the unregister reflects that it was cleaned up

What I don't want is to have dangling objects (vertices and edges) left over somewhere, and it would be less ideal to have to manually go through the graph and delete things.

[vsoch]

This tool appears to be deprecated so I wound up using standard runtime and then manual GC, and it does look like we are cleaning up. I'll keep this in mind moving forward.

Renaming the title here to reflect adding an unregister function to delete a particular subsystem cluster component.