[CLEANUP] Backend: Remove IE7/6/5 support

There are some places where there is special treatment for IE7
and below which can now be removed safely.

Resolves: #60528
Releases: 6.3
Change-Id: Ic942a499d1663672a6ac80966621010705e755be
Reviewed-on: http://review.typo3.org/31761
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Susanne Moog <typo3@susannemoog.de>
Tested-by: Susanne Moog <typo3@susannemoog.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>