ubi: Remove unnecessary struct declaration

struct ubi_wl_entry is defined at 178th line.
The declaration here is unnecessary. Remove it.
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/ubi/ubi.h

@@ -388,8 +388,6 @@ struct ubi_volume_desc {
 	int mode;
 };
 
-struct ubi_wl_entry;
-
 /**
  * struct ubi_debug_info - debugging information for an UBI device.
  *