mtd: nand: remove few tiny page NAND bits

NAND flashes with 256 bytes NAND pages are so old that probably do not exist
any more. Let's remove few related pieces of code and forget about them
forever. The assumption will be that 512 bytes NAND page size is the minimum
possible.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>