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 10f22dde556d1ed41d55355d1fb8ad495f9810c8

Signed-off-by: Gary Hade <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/mm/init_64.c

···
 		max_pfn_mapped = last_mapped_pfn;
 
 	ret = __add_pages(zone, start_pfn, nr_pages);
-	WARN_ON(1);
+	WARN_ON_ONCE(ret);
 
 	return ret;
 }