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>

mm/memory_hotplug.c

@@ -374,10 +374,6 @@ void online_page(struct page *page)
 		totalhigh_pages++;
 #endif
 
-#ifdef CONFIG_FLATMEM
-	max_mapnr = max(pfn, max_mapnr);
-#endif
-
 	ClearPageReserved(page);
 	init_page_count(page);
 	__free_page(page);