x86/cpu: Remove obsolete aperfmperf_get_khz() declaration

aperfmperf_get_khz() was replaced by arch_freq_get_on_cpu().
The remaining declaration in the header file is no longer used
and should be removed.

Fixes: f3eca381bd ("x86/aperfmperf: Replace arch_freq_get_on_cpu()")
Signed-off-by: Junxiao Chang <junxiao.chang@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Link: https://patch.msgid.link/20260606021514.1433619-1-junxiao.chang@intel.com

arch/x86/kernel/cpu/cpu.h

@@ -75,7 +75,6 @@ static inline struct amd_northbridge *amd_init_l3_cache(int index)
 }
 #endif
 
-unsigned int aperfmperf_get_khz(int cpu);
 void cpu_select_mitigations(void);
 
 extern void x86_spec_ctrl_setup_ap(void);