[BUGFIX] Decouple GeneralUtility::rmdir from Environment::isWindows()

To be able to call rmdir without an initialized Environment instance,
the check if a Windows system is used, is decoupled. This allows to
call GeneralUtility::rmdir e.g. in typo3/testing-framework.

Resolves: #86655
Releases: master
Change-Id: Ica1feafbd9a07005c4b653ef2fe1153fba4b9417
Reviewed-on: https://review.typo3.org/58631


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>