Bugfix: ServerOcclusion - clean up external references when BaseNetworkable is destoyed
Dobbie's free, this is the last bug I could theorycraft. There's a small window before occlusion would update and unsubscribe where a teleport then destroy could leave a dangling set of references, but this doesn't happen because of unnecessary SetParent being called when BaseEntity is destroyed on the server(but this should be fixed separately)
Tests: not testable