configs: Remove SF_DUAL_FLASH

SF_DUAL_FLASH claims to enable support for SF_DUAL_STACKED_FLASH and
SF_DUAL_PARALLEL_FLASH. But, in current U-Boot code, grepping for above
enums yield no user and therefore support seems to be incomplete. Remove
these configs so as to avoid confusion.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com> #zynq-microzed