Fix cache cleanup for phpThumbOf-generated images

Filenames for cached images created by phpThumbOf typically don't begin with 'phpThumb_cache', so [this line](https://github.com/modxcms/revolution/blob/develop/core/model/phpthumb/phpthumb.class.php#L660) in phpthumb.class.php prevents phpThumbOf's [cleanCache() method](https://github.com/splittingred/phpThumbOf/blob/develop/core/components/phpthumbof/model/phpthumbof/phpthumbof.class.php#L505-L507) from doing anything useful.  It just wastes time listing all the cache files without actually deleting any of them.

Also the phpThumbOf cache directory has a blank index.html file in it for security reasons which shouldn't be deleted.