mtd: Kill the OF_MTD Kconfig option

Commit d48f62b9 ("mtd: nand: move of_get_nand_xxx() helpers into
nand_base.c") removed the drivers/of/of_mtd.c file but did not remove
the associated OF_MTD Kconfig option.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>