Check to see if ThreadDestroyListLock can be removed

There is some (very old) code in jlib maintaining a list of threads being destroyed (ThreadDestroyList).

It claims to be avoiding a race condition between join and thread destruction, but it is not clear what the race condition is.  I suspect it is something to do with synchronization between the alive flag and the stopped semaphore, but may not even exist any more.

Worth revisiting to see if it can be removed.