cpu: Remove unused variable

GCC warns us about:

 kernel/cpu.c: In function ‘take_cpu_down’:
 kernel/cpu.c:200:15: warning: unused variable ‘cpu’

This variable is unused since param->hcpu is directly
used later on in cpu_notify.

Signed-off-by: Dhaval Giani <dhaval_giani@gmail.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1290091494.1145.5.camel@gondor.retis>
Signed-off-by: Ingo Molnar <mingo@elte.hu>