[BUGFIX] Do not load ExtJS again via AJAX

Resolves: #68396
Releases: master
Change-Id: I8ccdcd6466b3367503ae7c8385015f7e34994b4f
Reviewed-on: http://review.typo3.org/42080


Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>