Fixes #8326: deletion of directories on S3

Since there are no real directories on S3 the previous OC\Files\Storage\AmazonS3::unlink did not work as expected. The method now determines if the supplied path is a directory and then executes OC\Files\Storage\AmazonS3::rmdir. rmdir has been improved to use native sdk-methods, which means that recursion is no longer necessary.