[!!!][TASK] Remove deprecated LanguageService-related settings

Removed configuration option:
* $GLOBALS['TYPO3_CONF_VARS']['BE']['languageDebug']

Removed public property:
* TYPO3\CMS\Core\Localization\LanguageService->debugKey

Removed public methods:
* TYPO3\CMS\Core\Localization\LanguageService->getLL
* TYPO3\CMS\Core\Localization\Locales->getIsoMapping

Resolves: #101037
Related: #100963
Related: #100721
Related: #99531
Releases: main
Change-Id: I019aa6fa6507927cde197c38540643271cad83ee
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79322


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Benni Mack <benni@typo3.org>