[TASK] Remove specials for IE6 from t3skin

IE6 Supports will be dropped in 4.6...
There are some special CSS files, 2 php classes and sprites as GIF Icons.
Remove the special IE6 handling in t3skin

Change-Id: I2151004f574fb2b3891ff28914814a94a3eb97e0
Resolves: #M17805
Reviewed-on: http://review.typo3.org/949


Reviewed-by: Tobias Liebig <mail_typo3.org@etobi.de>
Reviewed-by: Jigal van Hemert <jigal@xs4all.nl>
Tested-by: Jigal van Hemert <jigal@xs4all.nl>