UBI: ubi_eba_read_leb: Remove in vain variable assignment

There is no need to set err, it will be overwritten in any case
later at:
        if (scrub)
                err = ubi_wl_scrub_peb(ubi, pnum);

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