clocksource: sh_cmt: use remove_irq() and remove clockevent workaround

Update the sh_cmt driver to make use of recent irq and clockevent changes:
 - use remove_irq() together with setup_irq()
 - remove mult workaround since WARN_ON() now has been moved

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>