[TASK] Remove hardcoded check on typo3/sysext/

When running TYPO3 with custom components (subtree), the typo3/sysext/
directory is not always necessary, and the hardcoded check is obsolete.

Resolves: #82622
Releases: master, 8.7
Change-Id: I3e939c80fff30d82dd4eb624c2f4bc5db1696809
Reviewed-on: https://review.typo3.org/54230


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>