ubi: Remove useless code in bytes_str_to_int

As a local variable, "endp" is neither refered nor returned
after this line "endp += 2", it looks like a useless code,
suggest to remove it.

Signed-off-by: Chengsong Ke <kechengsong@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>