xen/acpi: Remove the WARN's as they just create noise.

When booting the kernel under machines that do not have P-states
we would end up with:

------------[ cut here ]------------
 WARNING: at drivers/xen/xen-acpi-processor.c:504
 xen_acpi_processor_init+0x286/0
 x2e0()
 Hardware name: ProLiant BL460c G6
 Modules linked in:
 Pid: 1, comm: swapper Not tainted 2.6.39-200.0.3.el5uek #1
 Call Trace:
  [<ffffffff8191d056>] ? xen_acpi_processor_init+0x286/0x2e0
  [<ffffffff81068300>] warn_slowpath_common+0x90/0xc0
  [<ffffffff8191cdd0>] ? check_acpi_ids+0x1e0/0x1e0
  [<ffffffff8106834a>] warn_slowpath_null+0x1a/0x20
  [<ffffffff8191d056>] xen_acpi_processor_init+0x286/0x2e0
  [<ffffffff8191cdd0>] ? check_acpi_ids+0x1e0/0x1e0
  [<ffffffff81002168>] do_one_initcall+0xe8/0x130

.. snip..

Which is OK - the machines do not have P-states, so we fail to register
to process the _PXX states. But there is no need to WARN the user
of it.

Oracle BZ# 13871288
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>