mtd: drop CONFIG_MTD_CONCAT ifdefs

As MTD_CONCAT is becoming a part of mtd core, it's now meaningless
to to check for it in ifdefs. Drop such references from MTD code.

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>