x86/cpufreq: clean up stale powernow_cpufreq_init()

Remove useless declaration, eliminating a MISRA C:2012 Rule 8.6
violation: The routine itself was removed by following commit long time
ago:

   222013114 x86: Fix RevF detection in powernow.c

No functional change.

Signed-off-by: Sergiy Kibrik <Sergiy_Kibrik@epam.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/include/acpi/cpufreq/processor_perf.h

@@ -7,7 +7,6 @@
 
 #define XEN_PX_INIT 0x80000000U
 
-int powernow_cpufreq_init(void);
 unsigned int powernow_register_driver(void);
 unsigned int get_measured_perf(unsigned int cpu, unsigned int flag);
 void cpufreq_residency_update(unsigned int cpu, uint8_t state);