[FOLLOWUP][TASK] Remove ExtJS-related assets

Now unreferenced icons and CSS files have been forgotten to be removed.

Relates: #68075
Resolves: #83427
Releases: master
Change-Id: I96416220f56c8394104bd626f16e6215c651e8a8
Reviewed-on: https://review.typo3.org/55211


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Benjamin Kott <benjamin.kott@outlook.com>
Tested-by: Benjamin Kott <benjamin.kott@outlook.com>