[BUGFIX] FunctionalTestCaseBootstrapUtility throws warnings on Windows

If the FunctionalTestCaseBootstrapUtility::removeInstance() tries to
delete an existing symlink to a folder, the function unlink() is used.
This leads to a warning on Windows systems because unlink can only
delete files and symlinks to files.
This patch adds a rmdir() call for symlinks which works for Windows
system as well.

Resolves: #56892
Releases: 6.2
Change-Id: I2de757daea89a65a0a631c7592287e16218c1452
Reviewed-on: https://review.typo3.org/28371
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind