riscv: remove CONFIG_ARCH_MAP_SYSMEM from io.h

Commit ba1ed5b0 ("Convert CONFIG_ARCH_MAP_SYSMEM to Kconfig") clearly
defined that this option is available for SANDBOX (was also for already
removed NDS32). That's why there is no way how this code can be enabled
with current Kconfig layout for riscv.
Based on this removing this code.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>