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>

drivers/of/Kconfig

@@ -90,10 +90,6 @@ config OF_PCI_IRQ
 	help
 	  OpenFirmware PCI IRQ routing helpers
 
-config OF_MTD
-	depends on MTD
-	def_bool y
-
 config OF_RESERVED_MEM
 	depends on OF_EARLY_FLATTREE
 	bool