thermal: intel: powerclamp: Remove dead code for target mwait value

After conversion of this driver to use powercap idle_inject core, this
driver doesn't use target_mwait value. So remove dead code.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>