powercap: RAPL: remove unused local MSR define

Remove unused PLATFORM_POWER_LIMIT MSR local definition from file
intel_rapl_common.c. This was missed while splitting old RAPL code
intel_rapl.c file into two new files intel_rapl_msr.c and
intel_rapl_common.c as per the commit 3382388d
("intel_rapl: abstract RAPL common code"). Currently, this #define
entry is being used only in intel_rapl_msr.c file and local definition
present in this file.

Signed-off-by: Sumeet Pawnikar <sumeet.r.pawnikar@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>