mtd: drop MTD_CONCAT from Kconfig entirely

As there are no more dependencies on MTD_CONCAT or CONFIG_MTD_CONCAT,
drop this entry from Kconfig entirely.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Acked-by: Stefan Roese <sr@denx.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>