mtd: spi: Remove SF_DUAL_FLASH symbol from Kconfig

This symbol is not used anywhere in the code. Just enable in couple of
defconfigs but it does nothing that's why remove it.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>