mtd: nand: Remove unused chip->write_page() hook

The last/only user of the chip->write_page() hook (the Atmel NAND
controller driver) has been reworked and is no longer specifying a custom
->write_page() implementation.
Drop this hook before someone else start abusing it.

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