kirkwood: sysupgrade: drop unnecessary UBI to UBI logic

kirkwood: sysupgrade: drop unnecessary UBI to UBI logic

The purpose of this code seems to be to avoid issues caused
by partially overwriting an existing UBI partition, where some
of the erase counters would be reset but not the unmodified
ones.  This problem has been solved in a more generic way by
the UBI EOF marker. This ensures that any old PEBs after the
marker are properly initialized.  It is therefore unnecessary
to erase the whole partition before flashing a new OpenWrt
factory image.

Signed-off-by: Bjørn Mork <bjorn@mork.no>