Revert "UBI: use mtd->writebufsize to set minimal I/O unit size"

This reverts commit a121f643.

Unfortunately, this commit breaks UBIFS backward compatibility and
makes new UBIFS refuse older UBIFS-formatted media:

UBIFS error: validate_sb: min. I/O unit mismatch: 8 in superblock, 64 real

Thus, we have to revert this patch and work on a better solution.

Reported-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>