[DOCS] Avoid use of TYPO3_MODE checks in ext_localconf.php

As it's highly discouraged to check for global constants in
ext_localconf.php due to limited caching the example code
is updated to not propose this check anymore.

Resolves: #90974
Releases: master, 10.4, 9.5
Change-Id: I32e9d39d9a5a975986fd009ccf60a0fcf1a8fed0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/65511


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Jörg Bösche <typo3@joergboesche.de>
Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Jörg Bösche <typo3@joergboesche.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>