mtd: nand: remove unused NAND_OWN_BUFFERS flag

The last/only user of NAND_OWN_BUFFERS (cafe_nand.c) has been reworked.
This flag is no longer needed.

Suggested-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>