[BUGFIX] Fix set_no_cache() log level severity

The array key that is used to check if a Backend user is logged in
is fixed.

Resolves: #59495
Releases: 6.2
Change-Id: Ida6609c3bde998e17a0d9e9184adb48f227507f5
Reviewed-on: https://review.typo3.org/30663
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring