[FOLLOWUP][TASK] Remove migrated TYPO3_CONF_VARS/SYS/setDBinit

This patch removes the obsolete "setDBinit" option which was migrated
in TYPO3 v8.

Resolves: #82421
Releases: master
Change-Id: I96477e4dd7e151bf74caeafa7bdc38a1cde2f484
Reviewed-on: https://review.typo3.org/54255


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>