[Ticket/13009] Cleanup Tweaks CSS

Remove IE hacks for IE browsers that are no longer being supported and cleanup the remaining css

TESTING:
- [ ] IE 8
- [ ] IE 9


Ticket: https://tracker.phpbb.com/browse/PHPBB3-13009
PR Dependencies: https://github.com/phpbb/phpbb/pull/2504

PHPBB-13009