[TASK] Remove TYPO3_CONF_VARS BE/flexformForceCDATA

TYPO3_CONF_VARS BE/flexformForceCDATA is an ancient
setting introduced 2009 with #19338 in TYPO3 4.3 to
work around a weird behavior in libxml at that time.

It is the last usage of 'useCDATA' in GU::array2xml()
and some others. We can remove both.

Resolves: #102146
Related: #19338
Releases: main
Change-Id: I11c98c857a0fd85f1b3c9f66ae3b0896d7eec9ba
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81413


Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Garvin Hicking <gh@faktor-e.de>
Tested-by: Garvin Hicking <gh@faktor-e.de>
Tested-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>