[BUGFIX] Remove obsolete AJAX routes in EXT:backend

This patch removes obsoletes routes that were left from the ExtJS
removal.

Resolves: #83276
Related: #82426
Releases: master
Change-Id: I603022fee7bb8add882a058f1f000b47705a8df2
Reviewed-on: https://review.typo3.org/55015


Reviewed-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Tested-by: Jigal van Hemert <jigal.van.hemert@typo3.org>