powerpc/smp: No need to set_need_resched when getting a resched IPI

The comment in the code was asking "Do we have to do this?", and according
to x86 and s390 the answer is no, the scheduler will do it before calling
the arch hook.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>