clocksource: Get rid of clocksource_calc_mult_shift()

No one is using this, so encourage the use of
clocksource_register_hz/khz(), and drop this helper.

Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
[jstultz: tweaked commit message]
Signed-off-by: John Stultz <john.stultz@linaro.org>