nnUNet: Migrating to cpp filesystem library to delete temp dir

Summary:

• Migrating to filesystem library to from itk helper function. ITK delete directory throws error if folder doesn't exist. This was behaviour caused errors when MITK workbench was closed.
• Change nnUnet tool temp folder name

Test Plan: No error should occur when MITK is normally closed

Reviewers: kislinsk, O1 MITK Reviewer Group I

Reviewed By: kislinsk, O1 MITK Reviewer Group I

Subscribers: kalali

Maniphest Tasks: T29528, T29527

Differential Revision: https://phabricator.mitk.org/D797