mm: remove dependency on CONFIG_FLATMEM from online_page()

online_pages() is only compiled for CONFIG_MEMORY_HOTPLUG_SPARSE, so there
is no need to support CONFIG_FLATMEM code within it.

This patch removes code that is never used.

Signed-off-by: Daniel Kiper <dkiper@net-space.pl>
Acked-by: Dave Hansen <dave@linux.vnet.ibm.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>