[TASK] Typo3DbBackend: Drop non-functional code

array_fill_keys() used without function

Resolves: #47185
Releases: 1.3, 4.7, 6.0, 6.1
Change-Id: Ic6d11e4cfbd579f6847c8c30245933697ac3b5ce
Reviewed-on: https://review.typo3.org/19914
Reviewed-by: Andreas Wolf
Tested-by: Andreas Wolf