fix the delete all function

This patch fixes issue https://github.com/owncloud/core/issues/754 for the stable45 branch.
(Master will get a new file version handling which will make this function obsolete. )

I fixed some broken function calls and removed the user-id from some paths. I don't know why the previous version added the user-id in front of the given directory? The view already has the correct root directory and the directory given to the function should always be relative to it.

If I didn't miss anything the only app which calls this function is the files_versions app, which work with this patch. Still it would be good if someone with a deep oc_filestorage understanding can have a look at it, maybe @icewind1991,  @blizzz or @MTGap as the author of this class?