[BUGFIX] Fix deleting folders with subfolders

[cweiske]

Instead of passing the folder path (identifier) to the deleteFolder() method, the object of the subfolder was passed.

This led to the exception "Instances of Aws\S3\S3Client cannot be serialized".

Test:

1. Create folder "subfolder1"
2. Within "subfolder1", create "subfolder2"
3. Upload a file into "subfolder2"
4. Delete "subfolder1".