[ticket/12487] Use new `microtime()` parameter

PHPBB3-12487
[ticket/12487] Remove legacy code

https://tracker.phpbb.com/browse/PHPBB3-12487

PHPBB3-12487
[ticket/12487] Restore timezone comment in includes/startup.php

https://tracker.phpbb.com/browse/PHPBB3-12487

PHPBB3-12487
[ticket/12487] Remove unneeded constant definition

https://tracker.phpbb.com/browse/PHPBB3-12487

PHPBB3-12487
[ticket/12487] Update code

- Replace $user->lang[] calls with $user->lang()
- Remove unused global definitions
- Remove unused variables
- Update $user->img() calls

PHPBB3-12487