powerpc: Remove unused cpu_idle_wait()

cpuidle uses a generic function now. Remove the cruft.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Link: http://lkml.kernel.org/r/20120507175652.330322737@linutronix.de