[!!!][TASK] Remove legacy pages_language_overlay functionality

This patch removes any occurrences of "pages_language_overlay" in use
and enables the feature "unifiedPageTranslationHandling", which 
removes any SQL CREATE/ALTER statements related to "pages_language_overlay".

TCA for table "pages_language_overlay" is kept for the time being to have
"MigratePagesLanguageOverlayUpdate" still working.

Resolves: #87293
Releases: master
Change-Id: I94514f30696dcbc2ad3ae8900a6a25b41d2a7bd9
Reviewed-on: https://review.typo3.org/59296


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>