[TASK] Cleanup Extension Manager code

A new unnecessary function was introduced with
https://review.typo3.org/42283/. To have a cleaner code we should stick
to the functionality we already have and don't do think twice.

Resolves: #70101
Releases: master
Change-Id: I591ceb7a4e42b191aacb89add850fe996660d1e0
Reviewed-on: http://review.typo3.org/43560


Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>