microblaze: remove init_mm

Alexey removed the definition for init_mm from all architectures
but forgot microblaze, which was only recently added.
This fixes the microblaze build by dropping it there as well.

Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michal Simek <monstr@monstr.eu>