ramips: mt7621: disable CONFIG_MTD_SPLIT_FIRMWARE

It's no longer needed as all mt7621 devices use DT binding (supported by
upstream mtd code) for specifying "firmware" part format explicitly.

Signed-off-by: Rafał Miłecki <[email protected]>

target/linux/ramips/mt7621/config-4.14

@@ -187,7 +187,6 @@ CONFIG_MTD_NAND=y
 CONFIG_MTD_NAND_ECC=y
 CONFIG_MTD_PHYSMAP=y
 CONFIG_MTD_SPI_NOR=y
-CONFIG_MTD_SPLIT_FIRMWARE=y
 CONFIG_MTD_SPLIT_MINOR_FW=y
 CONFIG_MTD_SPLIT_SEAMA_FW=y
 CONFIG_MTD_SPLIT_TPLINK_FW=y