[TASK] Clean up EXT:impexp export folder after test run

Running the functional EXT:impexp tests causes the
EXT:impexp/Tests/Functional/Fixtures/Folders/fileadmin/user_upload
folder to grow larger. This is due to export files being created that
have not been cleaned up after the tests have run.

In addition, completely delete the default import/export folder
in ImportExport::removeDefaultImportExportFolder(). This
method is currently only used in tests.

Resolves: #94937
Releases: master
Change-Id: I55d821ec18689172a73e0f290f1e155fae64d3cb
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70683


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>