clocksource/drivers/h8300_timer8: Remove pointless irq re-entrant safe code

The current code assumes the interrupt function is re-entrant.

That is not correct. An interrupt handler is never invoked concurrently. The
interrupt line is masked on all processors.

Remove the chewing flags in the code.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>