[POWERPC] via-pmu: Kill sleep notifiers completely

This kills off the remnants of the old sleep notifiers now that they
are no longer used.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>