timerqueue: Remove never used function timerqueue_node_expires()

This function was introduced with commit 60bda037 ("posix-cpu-timers:
Utilize timerqueue for storage") but never used. Remove it.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240417140229.19633-1-anna-maria@linutronix.de