timekeeping: Remove TK_MIRROR timekeeping_update() action

All call sites of using TK_MIRROR flag in timekeeping_update() are
gone. The TK_MIRROR dependent code path is therefore dead code.

Remove it along with the TK_MIRROR define.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <jstultz@google.com>
Link: https://lore.kernel.org/all/20241009-devel-anna-maria-b4-timers-ptp-timekeeping-v2-24-554456a44a15@linutronix.de