Delete the temporary history file in Network.Cleanup()

The temporary files were already deleted via `deleteOnExit()`, but that’s not good enough: repeated simulations could still fill up the disk while JSim is still running. Now we make sure that at most one temporary file exists at a time.

(In Java 7, `deleteOnExit()` is obsoleted by a `DELETE_ON_CLOSE` flag, which will solve this in a better way.)