x86, apb_timer: remove unused variable percpu_timer

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Link: http://lkml.kernel.org/r/1356030701-16284-28-git-send-email-sasha.levin@oracle.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>