Rearrange and clean up code in time.el (Bug#40863)

Rearrange and clean up code in time.el (Bug#40863)

* lisp/time.el (world-clock, zoneinfo-style-world-list)
(legacy-style-world-list, world-clock-list)
(time--display-world-list, world-clock-time-format)
(world-clock-timer-enable, world-clock-timer-second): Move definitions
closer to 'world-clock' code.  Remove redundant :group args.

(display-time-mail-file, display-time-mail-directory)
(display-time-mail-function)
(display-time-default-load-average)
(display-time-load-average-threshold, display-time-day-and-date)
(display-time-interval, display-time-24hr-format)
(display-time-hook, display-time-mail-face)
(display-time-use-mail-icon, display-time-mail-string)
(display-time-format, display-time-string-forms): Remove redundant
:group args.