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.)

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
delete simulation file on close