ARM: sunxi: Remove init_machine callback

The init_machine hook is now at its default value. We can remove it.

Even though the sun4i and sun7i machines are nothing more than generic machines
now, leave them in so that we won't have to add them back if needed, and so
that the machine is still displayed in /proc/cpuinfo.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>