Fix: Cleanup nodes in Thread::LinkedList(T)#delete

[ysbaddaden]

We internally use Thread::LinkedList(T) to keep a list of all threads & fibers, so the GC won't collect them and know all the stacks to scan during a GC collection. I noticed that #delete doesn't cleanup the @previous and @next links of the T objects.

When we remove the thread or fiber from the list, it means they have terminated and the objects should get collected at some point, so I guess cleanup isn't very important. On the other hand, there isn't much harm to cleanup a couple pointers (quite the opposite).

In fact, while investigating a memory leak with PerfTools::MemProf.pretty_log_object_graph(io, Fiber) I noticed a number of Fiber objects kept references to the same set of other fibers, while with proper cleanup, the graph of objects remained clean & stable, maybe because the Fiber objects disappear much more quickly.