clocksource: Clean up clocksource_update_freq() functions

clocksource: Clean up clocksource_update_freq() functions

Remove the unused functions __clocksource_update_freq_hz() and
__clocksource_update_freq_khz().

Then make __clocksource_update_freq_scale() static as it is not used
from external callers anymore. Also clean up the comment accordingly.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Acked-by: John Stultz <jstultz@google.com>
Link: https://patch.msgid.link/20260504-clocksource-update_freq-v2-1-3e696fb01776@linutronix.de