Fix my previous change to cancel world-clock timer

Fix my previous change to cancel world-clock timer

* lisp/time.el (subr-x): Require when compiling.
(world-clock): Set 'kill-buffer-hook' buffer locally only.
(world-clock-update): Break out timer cancellation from here...
(world-clock-cancel-timer): ...to here, and don't rely on variable to
find the timer to cancel.
(world-clock-timer): Delete now superfluous variable.