UBI: remove data type hint from ubi-user.h

This finally removes the data type hint from the UBI ABI.
>From now on the "dtype" field will be ignored and must not used
anymore.

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