fs/ubifs: remove unnecessary new_valid_dev check

As currently new_valid_dev always returns 1, so new_valid_dev check is not
needed, remove it.

Signed-off-by: Yaowei Bai <bywxiaobai@163.com>
Reviewed-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>