microblaze: Remove wrong define CONFIG_SYS_FLASH_PROTECTION

CONFIG_SYS_FLASH_PROTECTION is defined twice. The second one
is outside the flash configuration context and we can remove
it safely.

Signed-off-by: Stephan Linz <linz@li-pro.net>
Signed-off-by: Michal Simek <monstr@monstr.eu>