[TASK] Tests: Remove backup of TYPO3_DB

All $GLOBALS are backed up by phpunit, no explicit backup
of $GLOBALS['TYPO3_DB'] is required.

Change-Id: I2859a5922d85a23fa8281382b1d78043ec7bd934
Resolves: #55846
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/27526
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn