fs: Remove magic field from partition_info_s

since it is wrong and impossible to return file
system magic number from the block or mtd layer.

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

drivers/mtd/filemtd.c

@@ -442,7 +442,6 @@ static int filemtd_ioctl(FAR struct mtd_dev_s *dev, int cmd,
             (FAR struct partition_info_s *)arg;
           if (info != NULL)
             {
-              info->magic       = 0;
               info->numsectors  = priv->nblocks *
                                   priv->erasesize / priv->blocksize;
               info->sectorsize  = priv->blocksize;