[BUGFIX] T3editor description loading

Due to the refactoring of the ajax registration a wrong
method is registered for retrieving the description in t3editor.

Resolves: #57708
Releases: 6.2
Change-Id: I4eff5fe9475151439ad4e3cae26c874f69694ac5
Reviewed-on: https://review.typo3.org/29237
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind