mtd: drop MTD_CONCAT dependencies from Kconfig

As MTD_CONCAT is becoming a part of mtd core, it's no longer necessary
to depend on it in Kconfig scripts. Drop such references.

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>