timekeeping: Remove the temporary CLOCK_AUX workaround

ktime_get_clock_ts64() was provided for the networking tree as a stand
alone commit based on v6.16-rc1. It contains a temporary workaround for the
CLOCK_AUX* defines, which are only available in the timekeeping tree.

As this commit is now merged into the timers/ptp branch, which contains the
real CLOCK_AUX* defines, the workaround is obsolete.

Remove it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250701130923.579834908@linutronix.de