[BUGFIX] Unit tests: Superfluous backup

Globals are automatically back-upped, no need to do that manually.

Change-Id: I40a6b046f6bb177100d72707a114de76ef466a55
Resolves: #52758
Releases: 6.2
Reviewed-on: https://review.typo3.org/24705
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn