xen: cpufreq: fix notifier block double registration

As a consequence of commit 49388f11d512bb92706ce ("x86/cpufreq: relocate the driver register function") the cpufreq CPU notifier was being registered twice. That resulted in bugs when trying to offline a CPU, as reported here:

https://www.mail-archive.com/[email protected]/msg41618.html

---
Cc: Jan Beulich <[email protected]>
Cc: Andrew Cooper <[email protected]>
Cc: "Wang, Wei W" <[email protected]>
Cc: Konrad Rzeszutek Wilk <[email protected]>

---
 xen/drivers/cpufreq/cpufreq.c |    1 -
 1 file changed, 1 deletion(-)

index 81a187b..6e666e4 100644
@@ -650,6 +650,5 @@ int __init cpufreq_register_driver(struct cpufreq_driver *driver_data)
 
     cpufreq_driver = driver_data;
 
-    register_cpu_notifier(&cpu_nfb);
     return 0;
 }