mtd: maps: remove check for CONFIG_MTD_SUPERH_RESERVE

Since (a few releases before) v2.6.0 there have been checks for
CONFIG_MTD_SUPERH_RESERVE. One check is still present. But a Kconfig
symbol MTD_SUPERH_RESERVE has never been added. So a few lines of dead
code can be removed.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>