powerpc/time: Remove unnecessary sanity check of decrementer expiration

The clockevents code uses max_delta_ns to avoid calling a
clockevent with too large a value.

Remove the redundant version of this in the timer_interrupt
code.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>