thermal/intel_powerclamp: Remove set-but-not-used variables

In poll_pkg_cstate() function, the variables jiffies_last and
jiffies_now are set but never used.

This has been detected by building the driver with W=1:

drivers/thermal/intel_powerclamp.c: In function ‘poll_pkg_cstate’:
drivers/thermal/intel_powerclamp.c:464:23: warning: variable
‘jiffies_last’ set but not used [-Wunused-but-set-variable]
static unsigned long jiffies_last;
                       ^

Signed-off-by: Augusto Mecking Caringi <augustocaringi@gmail.com>
Acked-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>