[!!!][TASK] Remove language-related deprecated functionality

The global variable 'LOCAL_LANG' is now removed, as all labels
should always be fetched via LanguageService.

In addition, the following methods have been removed:
* Locales::initialize()
* LanguageService->getLabelsWithPrefix()

The following public methods and properties have now changed their visibility to protected
* LanguageService->LL_files_cache
* LanguageService->LL_labels_cache
* LanguageService->debugLL()
* LanguageService->getLLL()

The method LanguageService->includeLLFile() now only expects one argument

Resolves: #91475
Related: #91473
Releases: master
Change-Id: I617a2c11e5b1b052b5e0137e274d1f2aeb7b3b12
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64595


Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>