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>