[TASK] Remove obsolete FormInlineAjaxController method

Protected controller method getErrorMessageForAJAX() is
unused since v7 and can be safely removed.

Resolves: #96828
Releases: main
Change-Id: I39e1229c8600ced151ddafdb982167b9d2ba2975
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73421


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>