xen/smp: Remove unnecessary call to smp_processor_id()

There is an extra and unnecessary call to smp_processor_id()
in cpu_bringup(). Remove it.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>