x86: remove debug code from arch_add_memory()

Impact: remove incorrect WARN_ON(1)

Gets rid of dmesg spam created during physical memory hot-add which
will very likely confuse users.  The change removes what appears to
be debugging code which I assume was unintentionally included in:

  x86: arch/x86/mm/init_64.c printk fixes
  commit 10f22ddeSigned-off-by: NGary Hade <garyhade@us.ibm.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/mm/init_64.c

@@ -858,7 +858,7 @@ int arch_add_memory(int nid, u64 start, u64 size)
 		max_pfn_mapped = last_mapped_pfn;
 
 	ret = __add_pages(zone, start_pfn, nr_pages);
-	WARN_ON(1);
+	WARN_ON_ONCE(ret);
 
 	return ret;
 }