powercap: intel_rapl: Remove unused field in struct rapl_if_priv

After commit f1e8d756 ("powercap/intel_rapl: enumerate Psys RAPL
domain together with package RAPL domain"), the platform_rapl_domain field
is not used anymore. Remove it from rapl_if_priv structure.

Fixes: f1e8d756 ("powercap/intel_rapl: enumerate Psys RAPL domain together with package RAPL domain")
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>