Followup EZP-22917: make eZFSFileHandler::fileDelete() less verbose

After e9bfd8cea8, `eZDir::recursiveDelete()` has been made verbose, and
clearing all caches results in such error message:

>[ Jul 04 2014 16:33:47 ] [] eZDir::recursiveDelete:
>The path: var/cache/ini is not a folder

Mimic what is done in `eZFSFileHandler::delete()` by checking if the path
given to `eZDir::recursiveDelete()` is a directory.