[CLEANUP] Remove IE6 fixes for RTE htmlarea

As IE6 is not supported for the TYPO3
backend anymore, the IE6-specific styles
for HTMLarea can be safely removed.

Resolves: #60770
Releases: 6.3
Change-Id: I11e91f705c49be9f2ead48c0d741242acda6cbb6
Reviewed-on: http://review.typo3.org/31978
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>