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>