[TASK] Remove last relics of IE6 backend support

[TASK] Remove last relics of IE6 backend support

Remove the last occurrences of IE6 specific code.
Resolves: #26274

Change-Id: I82e86c5b8ecc14d6592871a2f86d6c50d69eed48
Reviewed-on: http://review.typo3.org/1723
Reviewed-by: Peter Beernink
Tested-by: Peter Beernink
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn