tick/nohz: Remove unused tick_nohz_full_add_cpus_to()

This function isn't used anywhere. Remove it.

Signed-off-by: Alex Shi <alexs@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250410092423.9831-1-alexs@kernel.org