UBI: Don't read back all data in ubi_eba_copy_leb()

Drop this paranoia check from the old days.
If our MTD driver or the flash is so bad that we even cannot
trust it to write data we have bigger problems.

If one really does not trust the flash and wants write-verify
she can enable UBI io checks using debugfs.

Signed-off-by: Richard Weinberger <richard@nod.at>