clocksource: Remove unused clocksource_change_rating

clocksource_change_rating() has been unused since 2017's commit
63ed4e0c ("Drivers: hv: vmbus: Consolidate all Hyper-V specific clocksource code")

Remove it.

__clocksource_change_rating now only has one use which is ifdef'd.
Move it into the ifdef'd section.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20241010135446.213098-1-linux@treblig.org