sparc64: remove online_page()

The identical online_page() implementations from all architectures got
moved to mm/memory_hotplug.c - except for the sparc64 one that even was
dead code due to MEMORY_HOTPLUG not being available there.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>