Replace deleteAll call with unlink call on rename

Replace deleteAll call with unlink call
    
The method deleteAll() doesn't officially exist on the Storage class as it's not defined on the interface, which means it fails on the Quota storage wrapper and might fail on some external storage classes.
Also, this here is the only use case for that one method.

I suggest to keep "deleteAll" in the OC6 backport in case it was used by third party apps, even though it's not in the public API.

Please review @karlitschek @icewind1991 @schiesbn