timers: Remove unused inline funtion debug_timer_free()

There is no caller in tree, remove it.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20200909134749.32300-1-yuehaibing@huawei.com