clockevents: Remove unnecessary unlikely()

WARN_ON() and WARN_ON_ONCE() already contains an unlikely(), so it's not
necessary to use unlikely.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20181104023104.2572-1-tiny.windzz@gmail.com