[TASK] Remove warning if IE6 is used on backend login screen

When IE6 is used a message is shown that IE6 is not supported.
IE6 support was already dropped since TYPO3 CMS 4.6 so we
now remove this warning.

Resolves: #60640
Releases: 6.3
Change-Id: Ib964fef63fcd987eef4cc04264fff0c7cc761d47
Reviewed-on: http://review.typo3.org/31883
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Andreas Fernandez <andreas.fernandez@aspedia.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>