Improve action_delete_attachment

My log files were piling up with file not found exceptions from the buckets. This is because I have multiple sizes set up as the same dimensions. When one size is removed the other size still points to the same size and throws an exception when removed.

To fix this I added the following check to the sizes loop in action_delete_attachment:

            if ( ! in_array($path, $paths) )
            {
                array_push($paths, $path);
            }

Could you please add this improvement to the codebase?